Honestly, I think Custap is a problem. Custap + Endure is just too strong for too little opportunity cost. It allows you to negate the flaws of a pokemon while keeping the benefits. Rhyperior is a perfect example. Rhyperior gets the benefits of his stats, absurdly high HP, Defense, and Attack that allow him to win against physical attackers. However, his low speed and special defense balance him out. Custap + Endure allow him to negate both his low speed and special defense by tanking any hit and then getting free priority on a 150bp stab, which gives him matchups that he has no right winning. Drampa is an even better example as he even gets the exact same power as specs, can’t be one shot, and gets free priority on a +1 hyper beam coming from 135 Special attack. Literally specs but better. It hampers teambuilding by making speed less of a factor in viability.
It adds a ton of unfair mindgames. Obviously, guessing sets is a key part of 1v1. However, as Mimikyu perfectly illustrated, and Dragonite to a lesser extent, having sets with different counters and checks creates unfair situations. Custap does that for every pokemon while also getting rid of the normal answers that things like sturdy have. Endure lets you survive everything at any health, so you can't just hit it with a non supereffective move and then kill it, you have to play endure mindgames to win because they can use endure at any time and if they get it off they probably win. You can't use a multi hit move, because endure doesn't give a shit about that. It completely stone walls the normal counters for moves and abilities that let you always take a hit. It boils down the offensive counters to "Be able to OHKO and always take a hit", which is not healthy. Now, that description doesn't include stall/defensive mons, but Custap users can overcome them too. All they need is a boosting move. In fact, boosting move + custap is actually kinda broken. Not only does it give you the W against a ton of defensive pokemon, but it allows you to do so without sacrificing any offensive matchups.
Previously, I said that Custap hurts teambuilding and specialization. I’d like to expand on that. Custap takes away from the importance of Evs. That might sound dumb, but think about it. Custap allows you survive any move and then always hit first, so you don’t have to bulk to survive something. (Outside of specific cases in which it's useful.) It takes away the importance of defense and speed for the custap user, and warps the complexity of matchups into “Can I one shot you?”. It gives you too much reward for basically no risk. Why run scarf when you can always outspeed and not be locked in? Why be band when you can just run a boosting move and win against even more things without sacrificing any matchups? (If you’re slower, and already won, the just boost then endure. If you’re faster and won, boost then attack.) Why be life orb when custap does it better? Priority is one of the strongest tools a move can have, which is why the standard power of priority moves is 40. Custap breaks that balance in a meta where one attack usually wins the game.
Even though I think Custap is a problem, and a big one at that, I don't think it warrants a quickban. I say we should wait for the meta to develop more before we ban or suspect it. I made this post to gives some points on why Custap is unhealthy, not to call for a ban right now.