Assassin isn’t very good when comparing his versatility with other classes. Here are my 3 reasons why.
General Play
His dps is decent but the trickster is the way to go when going for dagger dps anyways. He doesn’t have any protective abilities so his rushing is the worse of the three dagger classes if he is meant to rush at all.
Ability
His ability is inconvenient. It is an ability that deals damage over time but there’s no real pay off to it so its just sluggish version of any damage dealing ability. Yes you can stack poison and applied poison can hurt invulnerable enemies but dealing bursts of damage with the Wiz or Samurai when the enemy is vulnerable is just as effective considering poisons can only be applied to vulnerable enemies anyways. Also there’s a cast time compared to a wiz and samurai’s instantaneous one.
No Support
Doesn’t have anything unique to be supporting a team of players. No heal, no debuffs, no buffs, no quirky ability for protection or rushing, just deals damage which if you are going to do, just play Wiz.