You don't actually mention in your post what your not happy with?
Firstly you mention what you have accomplished in previous games you have played which is great but this is a different game, with new mechanics not seen in the other mmorpg uve mentioned.
Now you mention why would a team take a assassin over an operative;
Operatives have burst sure but in the competitive arena that's about it.
Our healing is poor as we don't speced into it, and our resource goes fast if we attempt to heal any one for a more than a few seconds, its also incredibly easy to lock down our healing as our second heal requires additional resources from ta.
We can be taken out from.fights very easily in hutball there is no sprint or charge abilities, so if we get a poor position getting pushed off ramps means we are out of a fight for 30+ seconds. A decent team can also keep an .operative out of a fight for a long time with appropriate cc, knockback. Also if we.get rooted we cant use our snipe or any cover moves.
Our peeling moves are limited to 10m range, and have no stealth peals if Target is in combat, no pushes, no pulls and no mitigation to offer our team m8.
I have not listed here things like stealth or vanish because these things as an assassin you also have.
But you do have taunts and guard to help ur team mitigate damage, you do have a sprint to escape ontop of vanish or to close a gap/get over a fire pit.
Your range attacks are not effected by any root on you.
I would also like to point out that kills and total damage in a warzone also mean little, I personally have done 400k dmg 100k healing in hutball and my team still lost 6 - 0.