Eh, invulnerable can certainly be survivable enough, but it only rivals granite vs S/L. Not a build master but quick check in mids and wasn't able to get other resists past 55% for inv, where granite alone brings everything but psi over 80% with some simple franken-slotting, and easy reach to softcap. Add rooted and you have better regen and better -def/-end resist (although no -slow, plus the built in -slow/-recharge/-dam in granite).
Granted, it's usually not necessary unless you want to go afk to make a sandwich in the middle of a full mob of +4 spawn from DA missions, but kings of taking a hit should be acknowledged.