The assassin has a major problem and it is its existence itself. I don't know about mir 2 but mir 3 didn't need any additional class.
To be honest it's not the Assassin that's broken it's multiple things that results in the class becoming broken e.g Mana Scaling, Critical PVE, Standard Critical hits dealing double damage, Pig stats, Master refinement, Special Refinement, Edge System, % refinement, High stats on gear, Critical Chance on items, and Luck 10.
All these things build on top of the classes not just the Assassin until they break the game. I've been playing around with a GM who is working on a server lately and some of these systems have been removed, some have been changed and the classes them self have been balanced perfectly for PVE and seem fine PVP so it's not impossible to balance the classes but the more stats added to the game the more unbalanced it becomes.
I think when Jamie first created the files they were pretty well balanced it's just the addition of all these things that have pushed the damage of classes to ridiculous heights. Personally I loved playing Zircon it was only when the stats on items started to jump a bit too much and the edge, and special refinement got added that I could see the server not remaining somewhat balanced much longer and decided to walk away.
Looking forward to seeing what Jamie creates next though.