Assassination is viable, but it's the worst of the 3 specs.
Outlaw > Subtlety > Assassination
(few scenarios where Sub is actually better than Outlaw)
The gearing process is tedious, and more so annoying than other brackets.
(If you want to have a better understanding as to why it is annoying, refer to this link
https://xpoff.com/threads)
But you should have no complaints about damage, regardless of spec.