I only own the incinerator, but they just seem and feel kinda pointless with the mechanics this game has. Hear me out before throwing down explanations since it'll probably help you tailor your advice to my needs:
Sustain tanks can just be run away from if they do start winning a skirmish, and they can't well defend any of their low health allies from hit and run burst damage since you can't get a kill if people are just running. Unless you've got a scout buddy with you to chase people down, you're kinda useless. If you really depend on having faster mechs with you that much you may as well just play one yourself.
Burst tanks on the other hand don't do well in sustained skirmishes, which defeats the purpose of having a bunch of health in general and is inevitably just going to make you regret playing something slow.
The only ones which really look good are the vanguard and the grenadier since you can get point-D with a grenade launcher, a pretty nasty sounding weapon combination that's unavailable on anything else. Incinerator trails close behind since the gattling and fireballs are fairly similar in practice even though you manage resources differently.
Regardless, I still don't really see the incentive behind playing them. Whenever I see heavy mechs, unless they've got a player behind them who's just really good in general, I always see them as bonus points, and conversely when I'm the one in a heavy mech I too just feel like an easy bonus points. When they hang back I never think "dam, I can't chase this guy because there's a heavy mech there," and when they move in I never think "dam, this giant slow pile of health has prevented me from doing something". They don't seem a threat when they hang back, nor when they march in. Either hit and run the smaller targets they're hanging back to cover them, or cornerpoke the big guys themselves when they march in to attack. What do you add to the team?