able to clear content doesnt mean they are fine, go on the shadow class forum, they will clarify that for you
Being in used for progressions by top guilds does mean that they're fine for top level content, else these guilds would choose to take the other tanks. I have level 55 characters of all 3 tanking classes and choose to take my assassin for progression.
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.
I think the self heal from dark charge should be based off max hp to ensure they scale fairly.
Everyone complaining about sin tanks mitigation levels need to understand they fill a more utility role than straight up tank and take less damage like other tanks.