I've been literally thinking about this for a long time and I do think if they let them Dynamax, some of them would literally be broken tier. I think the offensive ones especially would be extremely difficult to deal with. In the right conditions, it's too easy for it to spiral out of control and really hard to stop, especially considering how easy it is for them to boost their stats.
For this reason, I think if they really want a healthy meta, it's best for them to not be able to Dynamax. For the time being though, this is speculation, because who knows what will actually happen...
One possibility is a nerf: Beast Boost does not work when Dynamaxed.
That being said, we already have Gyarados, Pinsir, Heracross, Scrafty, and Krookodile available, all of whom have Moxie and can Dynamax, and all three of whom have Attack as their highest stat. None of them are overpowered. Gyarados is the only one even close to out-of-control. The Ultra Beasts I can see becoming problematic with Beast Boost are Pheromosa, Kartana, Nihilego, Blacephalon, and Naganadel, all of whom are because they are speedy Pokémon with at least one low defense stat. Maybe Xurkitree due to having a monster Sp. Attack stat and only one weakness. However, we feared Dynamax Barraskewda before
Sword and Shield came out, and indeed, that fish was terrifying when you weren't prepared for it--but people DID find ways to fight it eventually, and now it's fast becoming irrelevant.
And if all else fails, there's always Focus Sash Malamar to go Topsy-Turvy on them.
Yes sorry if I wasn't clear captain jigglypuff , I'm taking about competitive Pokémon battling. Doubles specifically but could also be valid for singles. But raids can be fun though.
I actually think tapu bulu might actually finally be viable this gen. It gets grassy glide and play rough!
Tapu Bulu has always been viable, though he did trail behind the other three for the entirety of the generation. Definitely though, if he gets Grassy Glide, he'll catch up with the others or even exceed them.
The Tapus: "We are the legendary guardians of the Alola islands, we have power over the elements and will use terrains to increase our power and....wait, what's that sound?"
Ferrothorn with Steel Roller: "KEEP ROLLIN' ROLLIN' ROLLIN' ROLLIN'"
And Scolipede, who can outrun any of them after one turn with Protect--or use Steel Roller right away if you think they expect a Protect coming!