i may just be bad but i don't like Dynamax at all, at least in singles.
You are pretty much giving massive bulk, massive offense AND FREE BOOST all at once while dealing damage. Flying and Fighting moves are pretty much the worst to have, but weather ones are also quite nasty.
You named Gyarados. Dragon Dance, Water Moves for Weather Boost, Flying Move for Speed Boost and Moxie is already a pain to stop without using your Dynamax or some specific counters, depending on what the third attack is (Rotom W dies to Power Whip, Electric Types to EQ, Ferro is probably the most solid check, altho there are madmans with Rock Smash for the sweet Atk Boost and anyway decent damage after dynamax).
How to stop it then? Let it kill something and then send in your Ditto. GG EZ THX 4 FREE BST
Thanks to all the boosts going around with Dynamax Ditto has seen a rise in usage that i don't remeber since the introduction of Imposter. In itself it's not a terrible thing, if it wasn't that after stealing your buffs, with a Scarf to grab your balls, it can just Dynamax again with your Pokemon to get pretty much the effects of 2 Dyanamax in a match.
Another thing that i really hate is that Encore doesn't carries over the Dynamax, just like it did for Z Moves. Nerfing the move quite a bit. You encore me on a set up move? Too bad for you.
Outside of that, there are quite a bunch of new cool introductions, most new pokes are pretty cool (except for Darmanitan) and the new Items are really interesting
And then there is Moody Cancer, please ban.