I don't see a problem, you're reducing the damage you "would" take, a lv5 sentinel will be better than a lv1, and it also means now the weapon that's most effective against you is actually weaker than other weapons.

Have you seen assault rifles vs everything? Low TTK bruv, everything dies quickly, but that extra 10% on top is brutal. Am using the RR currently and when I come up against a heavy, it seems like forever melting the shields but once he's into armor he dies. the AR(Plasma Rifle) is the opposite, I'll rip into shields and then armor takes longer. These resistances would make the Sentinel better suited against weapons that are meant to drop it easily. instead of 110% damage, you'll take 95% damage, it's a fair buff.