Dynamax is about to get banned from 6v6 Smogon. At least, some changes were made in prevision of the potential future ban. I learned the hard way today that Choice items were disabled once you pokemon were dynamaxed.
In my case, I discovered this new feature with my Scarf-Ditto versus a Shell Smashed Blastoise (It was randbats, eveything hardcoded into the game is implemented there. Shell Smash Blastoise is not available yet). I dynamaxed my Ditto but the Blastoise was faster. It was a speed-tie. If you didn't know, if you had for instance a Choice Scarf, you can switch moves but the Speed Boost is removed.
Dynamax is ruling the OU metagame as we speak. There are currently 6 pokemons played in 30% of every team: Dragapult, Ferrothorn, Ditto, G-Darmanitan, Excadrill and Corviknight who can abuse it while being strong in other roles. Thanks to Dynamax and Choice items shenanigans, when G-Darmanitan was locked on a Ice-move, if the opponent sent Ferrothorn, G-Darmanitan dynamaxed, used Dynamax-Flare Blitz and won. And it's just one of many cases. Corviknight is literally immortal once it has set up with Bulk Up and Max-Airstream.
If you want a more detailed opinion, there is ABR's below, a respected top-player: