The complaints on the assassin forums are that they are spikey, this is countered by good players knowing how to use their cooldowns to maximum effectiveness.
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.