Choice Band is tempting on Hoopa-U, but I don't think it's its best item. Your intuition is correct - give it a slow pivot and let it wreck whatever it can. My concern here is the penalty of guessing "wrong" is much better than guessing correctly, and I find the flexibility of not using a choice item is helpful to make better midground plays. It really doesn't need more power, but needs a way to get and stay on the field consistently. My personal favorite item is still Assault Vest even after the memes of Specs Hoopa-U getting knocked off the OU ledge several generations ago. It's not quite fast enough to leverage Choice Scarf so might as well switch in against as many threats as it can just to get an attack off.
You're right that it's a little inflexible, but you can just get so many OHKO's with it than without. AV is nice, I've tried it before, and tbh it's probably better for a majority of teams, but building around it, I feel like banded is a better set considering AV or not you'll have to switch out whenever something remotely physically threatening comes in, and so the staying power is less useful during the beginning of the game. I think that even if band is worse, it's still a viable set, even if just for the surprise kills.
252+ Atk Choice Band Hoopa-Unbound Ice Punch vs. 0 HP / 4 Def Raging Bolt: 340-400 (86.9 - 102.3%) -- 93.8% chance to OHKO after Stealth Rock
- Takes 55 from T-bolt and then OHKO's it back
252+ Atk Choice Band Hoopa-Unbound Drain Punch vs. 252 HP / 4 Def Ting-Lu: 260-306 (50.5 - 59.5%) -- 82.4% chance to 2HKO after Leftovers recovery
252+ Atk Choice Band Hoopa-Unbound Knock Off (97.5 BP) vs. 248 HP / 252+ Def Corviknight: 211-250 (52.8 - 62.6%) -- guaranteed 2HKO
- Even Corv isn't a safe switch, I've had a couple games where they've come and then got 2HKO'ed by Knock
252+ Atk Choice Band Hoopa-Unbound Drain Punch vs. 4 HP / 252 Def Blissey: 622-734 (95.3 - 112.5%) -- 75% chance to OHKO
252+ Atk Choice Band Hoopa-Unbound Knock Off (97.5 BP) vs. 0 HP / 4 Def Ogerpon: 351-414 (116.6 - 137.5%) -- guaranteed OHKO
252+ Atk Choice Band Hoopa-Unbound Zen Headbutt vs. 0 HP / 4 Def Great Tusk: 398-470 (107.2 - 126.6%) -- guaranteed OHKO
252+ Atk Choice Band Hoopa-Unbound Knock Off (97.5 BP) vs. 252 HP / 40 Def Araquanid: 313-369 (92 - 108.5%) -- 50% chance to OHKO
- This one's far less important and less consistent but it's just funny getting their web setter off rip and now they're scrambling
I think that even with the inflexibility, it has surprising staying power since a lot of the time you can just click drain punch and gain 30% on whatever you just hit. I think it's a bit of an underrated set, definitely not some crazy anti-meta pick, but if you fit it with the right pieces I think you can get around the limitations of cb pretty easily.