Power suits and up are very decent at surviving plasma shots, except heavy or sniper. But in midgame and later, leaving your units open to fire is usually the worst idea no matter the armor, due to rockets, blasters and sectopods. Psi-puppets make far better "armor", anyway. Also you can make your own cover by punching walls in base assaults.
Assuming land alien encounters and slow basic agents, I'd say using anything besides mobility armor, such as synthsuits / assault / stormtrooper / prowler makes little sense. The purpose of heavier armors is to be vanguard or to cover positioning mistakes of lighter units. Heavy agents suck at this, as they usually lag behind the contact or are too far away from oopsie they're supposed to cover. This is HWPs's niche, as they're much faster. Advanced units such as spartans or infernals are also fast and thus can excel in whatever role/armor you'll give them.
In short - yeah, you can go fashion-xcomouls no problem.
