Even at 100%, there's still a chance for the debuff to fail.
Each debuff is applied separately and each one does the Accuracy versus Resistance check. And even if your Uugo has 700 accuracy versus an opponents 150 resistance, there's still about a 3% chance for the debuff to be resisted.
Gnishak in clan boss can work but he's a bit of a crazy champ for that because he could completely cover the debuff bar with bombs and poisons, so you need to be careful with that.
Having multiple champs with the same debuffs in clan boss is not the worst, especially dec atk is something you need to have placed for every turn. There are not a lot of champs who can do that on their own, so without an unkillable team you probably need to have two champs with dec atk in the team. I use Toragi and Sepulcher who both have it on their a1 and that's still not a 100%, but for me it's not a problem anymore because Gwyndolin just extends the debuffs every time she https://vlc.onl/
So you just have to figure out if Gnishak leaves enough room for other debuffs and how consistent you can place dec atk, no matter who the champs are. But Stag is generally not considered a great clan boss champ, but works of course if you don't have other options