My time with DM Hydreigon has been really limited due to IRL and so it's mostly been in the builder I've felt it, but both in game and in the builder it feels incredibly limiting. Basically everything I say here has already been said, but Hydreigon doesn't really have a true counter with DM access. It just comes in, threatens, and either nukes or will do something crazy like click NP/Sub/WU/FE/fill. Even the uber bulkies of the tier aren't safe since they can just be worn down via hazard chip or consecutive DM/coverage uses, or it can just surprise them with stuff like Flame/FBlast/Superpower, or if incredibly desperate it can just click U-turn and punish with a partner like Garchomp/Scizor/TTar/Infernape/Gallade/etc that can cause problems on their own anyway. The fact that every set it uses is already borderline is another factor, as you now have to win the guessing game of "is this Specs, Scarf, NP, or other" along with what coverage move they have. It's not even like Pokemon that could handle Draco/FBlast 2A exist anyway at large in the current metagame (which is like AV Metagross aka not that real and hates Flamethrower, P2 bc it can live a +2 FBlast before hazards and TP/Ibeam but its got a 1% usage rate, and Chansey but thats super mid off of bulkier builds tbqh), and the fact that it has more options than just this with really good bulk and even Roost for surprise healing on an offensive Pokemon is way too much. Dragon Pulse was much healthier when it could only use that, and it'll still be an extremely potent wallbreaker, stallbreaker, hazard setter/remover (kinda, this is more like a BO/Offense thing), and revenge killer without requiring the godliest of reads to safely pilot against.