He can be a main tank in KoTH maps
While he could be a main tank in payload maps, it would require you to build a comp that has high survivability or high mobility.
He is somewhere in between winston and roadhog. At my ELO and especially for me personally he works best with a shield tank, but he can also play Main decently with a Zarya who treats him like a quasi-Rein, or a really defensive DVA. Hog+Hammond can work occasionally but it’s very specific to the team and situation. I think he’s fun to play because he tanks a bit like the others but in a really unique ( and fast paced) way, Personally I think he plays a lot like Doom used to. All posted by a gold (been climbing fast to plats though!)
He is because he makes space for the team. He is the best initiation tank.
Funny thing though is that he can be played both as an off tank or main tank.
It depends on the comp, so basically hes both.
You can try to define what’s an off-tank or a main-tank, but it’s just an arbitrary term.
Also, although it’s arbitrary, it describes a ROLE in a team, and a role can be fullfilled by different characters depending on composition.
(For example, Zarya and Doomfist have been used as “main tanks” in certain composition in pro play)
What people usually want (and call “main tank”) in competitive is a shield tank, meaning Reinhardt, Orisa, or Winston
I think personally he’s a hybrid tank/dps.