Regarding the battle. 1st I wanna give my opinion on the Brothers and their showing. They are not popular characters. In fact, I venture to say they probably get more hate than majority of the characters in the manga.
I have seen them called fodder many times, and people praising Darui in his battle with them, while calling the brothers fodder. So, I wanted to give my thoughts on their battle, and why I don't feel they are fodder. I honestly think people hate a bit too hard on them.
Their fight wasn't the best, and I think Kishi really dropped the ball on their characters. But I don't think their performance was as piss poor as some fans make it, or that Darui was as grand as some make him. Of course Darui isn't weak. But he didn't seem stronger than the brothers.
Now, this is all purely opinion. So I am prepared for people to disagree.
Now, with the weapons and sealing. Raikage said Tobirama was almost killed. However, the weapons don't "almost" do anything. They either seal or they don't. Not counting the fan, of course.
They also surely had skill outside of those weapons that were good. And the vibe I got was they only use those weapons when they aren't serious.
Those weapons seem impracticable. The robe is fine, but it seems to constantly drain chakra, or chakra has to constantly be outputted while activated to keep the kotodama-squeezer activated.
Ginkaku had it around his neck with no consequence, but couldn't touch while it was on Kinkaku.
Then that big gourd is definitely not practical, having to be carried, thus Ginkaku only having 1 arm to fight with.
But even with that, it took Darui 2 hands to hold off a one-handed Ginkaku when they clashed shortly.
Ginkaku can slice a Zetsu clean in half with just one arm.
So he doesn't appear weak, and seems to be stronger than Darui, physically.
Ginkaku also doesn't appear to be slow. After sucking up Atsui, Samui cries out for him. But shortly after she cries out, Ginkaku is already behind her and taken her hostage.
Is he fast as Gai or Raikage? Of course I am not saying that. But still pretty quick on his feet. So I wager the brothers aren't slow. But there's middle ground between being a speedster and being slow.
Samui + Atsui both charge Kinkaku, but his reaction time and strength seem to be top notch, being he manages to knock them both away in one hit each.
Samui was trained by B, so I doubt she's slow.
When the coup d'etat happened, the Kage were left at death's door. Least the Hokage was. They may have killed the Raikage since they knew who the 3rd was. He, apparently, was appointed before they were killed.
Now again, the weapons seal or don't seal. There's no in-between. And since they were trying to take over, or working for someone who wanted to take over, I don't think they went there to screw around.
And going back to them messing around. Since they're already dead, they probably aren't as careful as if they were alive. That can make some Edo reckless. But, Darui manages to slice of Kinkaku's arm.
However, losing an arm hasn't shown to be lethal. Raikage, Naruto, Sasuke, and Jiraiya have all lost arms, and none of them died due to that.
Now, lets say Kinkaku's arm didn't grow back, he still had another to hold that sword, and we see him charge at Darui while he's stunned from the effects of the rope. Kinkaku could have easily severed Darui's head, rather than those words, and that'd have been the end of him.
I assume Kinkaku, being the bigger one, is stronger than Ginka. So if Gin can slice a zetsu body in half in one go, it probably isn't a problem for Kinkaku to lop off a head.
But apparently, that wasn't done because he wasn't serious, given that fact, plus that huge excited look on his face.
Honestly, the brothers look like they're having fun, rather than fighting with urge to slaughter right away.
So they may not have played that word game song-and-dance. Using the fan, and possibly the sword as a melee weapon at most, and their own skills, they may have managed to do that much damage to Tobirama and the Raikage, or they used Kyuubi power in conjunction with their attacks.
Again, they may have ambushed, but once they showed themselves, a fight was on I am sure. So, it was KinGin facing not only Tobirama alone, but the Raikage as well, and they apparently beat the hell out of both of them, if not killed the 2nd Rai, again, because they knew the 3rd.
When revived, seemed like they were just having fun toying with their enemies and weren't too rushed to kill right away, because not like that had anywhere to be or anything to get done.
So, just fight and play around. The only serious part is Ginkaku being sealed, and Kinkaku was pissed and serious then.
With the shrouds. I believe they can activate them any time they wish. Kakuzu says he saw Kinkaku use it when Ginkaku went down. But nothing said "he can only do that when Ginkaku dies."
That's just when it's prone to manifesting itself through his rage. Like when Naruto gets pissed, the chakra seeps through. But just because it manifests in his anger doesn't mean he can't tap into it, if he sees fit.
He did with summoning, and in his battle with Neji. Though after hurting Sakura, he seemed to shy away from using that power too much.
Kumo sources say the brothers emerged from Kurama's mouth looking like mini-kyuubi.
Both were alive, so nobody was raging then, and I am sure they had to be able to deactivate those forms.
Then there's some who bring up the brothers not being too smart. But where have they been shown to be stupid, exactly? From their showing, I won't say they're geniuses of the Shikaku caliber. But I don't think they're Beavis + Butthead / Yammy (bleach) either.
Least Kiinkaku has been shown as being perceptive / observant / clever. Whatever you wanna call it.
After being revived, he is the one that notices Darui's tattoo, and connects him to the 3rd, already being aware he's, at least, a decent fighter.
After seeing Atsui's word being hot, he promptly knows how to get him to say it.
When Darui lopped off his arm, he thought to kick it into him to nail him with the rope.
When Darui lectures the 2, Kinkaku is able to figure out, quickly, that Darui said his word and could be sealed.
When Darui escapes sealing, Kinkaku is shocked, but is able to deduce why things played out as they did.
All of these aren't that of a genius. But it certainly doesn't paint Kinkaku as an idiot. His intelligence seems average at the least, and average is not bad. It's normal, hence average. That's still not stupid, like some seem to think.
Now with KinkaKyuubi, many downplay him being "He lost to Ino, Shika, and Chouji." But really, as TenTen said, Kinka was tearing through that division like crazy, as noted by TenTen.
Kinkaku was the only summon there that had to have a special, hax, unbeatable sage tool there to seal him. They couldn't seal him through normal means. Raikage stating he was unstoppable in other way, aside from the pot.
And the pot sealed Gyuuki, a real Bijuu, so not shock Kinkaku lost to it.
And he only lost due to the hax of mind-transfer, forcing him to answer, otherwise, even that pot wouldn't be effective since he knows not to answer. Then people complain Ino could overpower him to do so, and have the nerve to compare Kin to Sakura, who could break free. A few things there.
1. Ino was only 12. I am sure she's improved after 4 years.
2. Ino used up too much chakra before she used that jutsu, as noted by Asuma + Kakashi. So the jutsu was less effective.
3. Sakura didn't break out instantly. Ino had her beat even despite the former two. However, Naruto calling out to her allowed her spirit to awaken and fight back. And even then a fight for dominance followed before Ino had to release.
With Kinkaku all it took was a few seconds for her to say she had him, and "yes" when he was called. The jutsu itself is very hax. Possible a Kekkei Genkai of the Yamanaka clan. So one losing to that doesn't = fodder to me. People just say such because it was Ino.
Then there's Kinkaku's will or brute strength, He, from what I recall, is the only one that's been able to outright break shadow-binding.
Though it's been a while since I read part one, maybe I missed something.
But far as I know Kakuzu got out, but that was because his arm was free and cut it off at the source. Had he not detached his arm, he'd have not been able to do that it seems, and apparently given how hard they were avoiding getting caught in it, they were taking it's power seriously. Unlike Kin who got caught, and broke it in a few seconds.
So, while the brothers didn't get a drawn out battle against other god Shinobi, I think they showed to be a threat, and I don't think they're as weak and stupid as some fans claim. And I don't think Darui was very impressive against them, since it seemed they were playing around while Samui, Atsui, and Darui were seriously going to take downs.
And really Darui's word changing was a pure fluke and luck on his part. So, he didn't beat the brothers on skill alone. And that makes his ending speech about stripping off the gold plating seem arrogant when he had 100+ ninja for back-up, 2 lucky saves, and fortune of having the most hax tool of the brothers there.