After carefully looking at the Assassin class I noticed the following:
Assassins have only (1) mechanism to get close the gap. Champions have (2) Leaps & range attack that slows the enemy, Templars have Congregation & Censure, Myrmidons have net pull & Colossus Smash. Knights have Chain Pull & Pursuit Propose: switching disengage to a pull mechanism similar to chain-pull. Making them at least pullers in larger group fights. If pulling is not an option consider giving a skill called "bolas" (Bolas - is a type of throwing weapon made of weights on the ends of interconnected cords, used to capture animals by entangling their legs)
Assassins self health is way too weak to even compete with other melee focused classes. For example: Knights can heal for 750 for stunning and knockdowns + Healing from blocking. Rangers can heal for 10% damage given. Myrmidons have berserk for sustain mechanism. Templar can heal by parrying and other mechanisms. Champions can heal when CC (there is ton of cc in game). Assassins can only heal for 450 for stunning ONLY. Propose: Give assassin a leeching poison ability "leeching poison". Healing over time for poisoning the enemy - Can only trigger for a certain amount of time.
I am not sure if the above proposed changes will "fix" the assassins but I am positive that it will improve their current situation. Assassins are not useful in large PvP battles and barely have survivability compared with other melee focused classes 1v1.