I agree that well played enemy is very hard to kill as a Sage. My opinion:
Sage has only light armor - 4 sec. stun, and you have 3/4 hp down. For examle BH after 4 sec. stun? Mayby dirty armor. You have no survival skill. Yes you have force armor, but it only prolong time do death by one hit.
Sages can't kill most meele classes - because meele also have cc skills(or like force leap skills) you are unable to kite any enemy. If enemy comes to you, you are death for sure.
Some cc skills are much stronger then sages' cc. So you can be more under cc(as a cc class:-) then your enemy. Force speed? Force leap, and you are death.
Survivability depends on cooldown of your cc. If your cc have cooldowns, you can't do anything against all classes. Anything.
Your dps is poor, against sniper you can't do anything - even you can't use mind snap, if enemy is in cover. Other range classes? You don't have enough dps.
For me is easy kill only spamming grenades BH, because if you use mind snap, they stand, doing nothing: what's happen, why my one button doesn't work?
I like playing as a Sage, but I think he is better in group as solo.