Just use the correct trinkets and know how to manage stamina and Soulvester is a pretty powerful toon. An extra max heart is way more powerful than you think, so he needs some sort of counter play (even if a little).
Don't forget that Soulvester has a lot more to his kit too. He has a more effective light during blackouts, is immune to the effects of ichor puddles (the covered screen, illness and even slowness debuffs), and is immune to long ranged attacks (which is pretty damn crazy if you ask me).
Sorry that Soulvester doesn't have 5 star skillcheck, movement speed, stamina, extraction speed and 1 star stealth + an active ability to heal all players up to max health at the cost of -12 tapes with a 0.2 pecosecond cooldown ¯\_(ツ)_/¯
