That's the issue for sure, their survivability. Even if they just perma guard with the current balance, they will still die. Most relevant bosses from fairly early on in the game start to magnify their damage quite a lot. They'll avaritia your party, creating a dark weakness, they'll use a magic attack that reduces their defense and magic defense, granting tactical advantage, and then next turn SOMEBODY is dead if you don't get a lucky miss through either a flash bomb or a far too expensive blessed bomb tactical advantage. The only way for anyone to not be a walking corpse is to simply be so overpowered you kill enemies before they can do too much.
The same goes for Slimey. Slimey was nerfed a lot over time. Increased MP cost, 'digestion', it takes a while before it begins to reach any kind of relevance outside of providing air bubble and storm once you reach nephlune.