Been one-shot by Terminate much lately? What about Force Scream + Voltaic Slash on Kel'sara's third phase?
Cooldowns do not solve the problem. This has been demonstrated MANY times on the tank forums, but I'll say it again here. Spike intervals are too frequent for CDs to always be up. Huge Grenade and Terminate are both excellent examples of this, since the only CDs which work on them have 2 minute cooldowns, while Terminate is used once every 40 seconds, and Huge Grenade is once every 80 seconds (give or take, I don't remember the exact interval). And on top of that, we only have one CD which is actually guaranteed (Resilience is 95%, Deflection is a 50% additive boost to a ~27% base, for a ~77% total chance).
Even if CDs did solve the problem, the fact that shadows *need* to use them while the other tanks don't and can thus save them for more opportune moments is in and of itself a significant balance issue.
I like my shadow too. I'm well aware of how to play it and how to maximize the value of my cooldowns and my self-heal. I'm not exactly new to the party. :-) Believe me when I say that the shadow issues are not just L2P related. The math is very clear on this. I know Suckafish takes an assassin tank to progression, and that does surprise me a bit, but it doesn't change anything about their current balance problems. Just because you *can* clear content with an assassin/shadow tank does not mean it is as easy to do so as it would be with a different tank class.
Terminate, yes, but i believe it's a badly designed move rather than the assassins fault. If they made it deal 90% of the target's health or something similar then it would keep that, you want to have the tank topped mentality without 1 shotting them. The kel'sara combo has never been an issue for me. Huge grenade u'd normally only take every other one and it doesn't do enough damage to warrant the use of a cooldown.
I don't use my cooldowns whenever I take a spike of damage due to bad rng I use it when i'm anticipating an increased period of damage. When you take spikes of damage due to bad rng it's up to healers to keep you up (one of the trade off from taking an assassin tank). The danger of death as an assassin is when a period of burst from the enemy lines up with bad rng, this is why you use your cooldowns here. There aren't any fights where you require more cooldowns than you have for these increased periods of damage.
The assassins are not even with the other tanks for their mitigation but you take them for their utility. There's nothing wrong with having a tank class that is designed to fill this style of role. If that's not their intent then that's a problem that Bioware should fix but doesn't stop them being equally viable for endgame content.