Basically, Z-Moves constitute an empty slot in Challenge Cup battles, since, upon usage, nothing happens. The only exceptions to this rule are the Z-Moves that are exclusive for a certain Pokémon, like Primarina's Z-Move, otherwise won't work. So please do something about this, either remove Z-Moves from CC tiers, or make a way that all Z-Moves can be used in CC tiers regardless of the requirements, because it's extremely annoying to end up with 3 Z-moves and 1 status move, it's literally impossible to attack.
I'm for removing Z-Moves from HCC since having the possibility of getting a moveset of literally unusable moves is just unfun. While some are usable, I don't think this justifies allowing all or even a small subset of the working ones to be used, especially when the ones that do function include moves like Extreme Evoboost, which will in almost all circumstances win the game for whoever has them. They should be limited to use with their respective Z-Crystals, like in any non-Hackmons tier.