I rolled a tank, and do OK as a tank. But there are aggro issues.
The two taunts work fine, but it is very easy to lose aggro because you simply cannot stay at the top in terms of DPS. So while you are hammering the boss, you should do OK. But do something else (like move for position, or hit shields, or move while tanking), you can lose aggro and have to re-taunt.
Without a target of target display, you have to watch the boss. If it turns to another player, re-taunt. I originally thought (incorrectly), that taunt would build aggro, it does not. It just puts you to the top (and then some plus whatever).
Overall though, mitigation is awesome on an Assassin tank. I just tanked my first hardmode (False Emperor) and was fine, no issues at all.
I have just learned to pay very strict attention to the boss, watch for lost aggro, make sure to re-taunt, and keep my dps up.
And yes, my dps blows. In pvp, its almost always dead last. But the good news is I am pretty damn hard to kill.