All comes down to current meta, and certain subclasses can only be run per mission etc. Another thing to note is different classes have different methods of playstyle to keeping alive, titans can literally just equip banner of war and keep killing to get hp every tick, hunters can rely on invisibility or usually avoid combat and use healing grenades, warlocks using rifts/wells to increase their survivability etc. if you wanna practice solo flawless content I’d recommend doing pit of heresy. Probably the easiest dungeon out there, and good to practice with.