RIven has no way to actually get instantly into your backline outside of flashing (unlike actual assassins), yes she has a lot of mobility but its not instant/long range mobility, her mobility adds up with time and she cannot just ignore frontline so she has to fight her way through, and thats why shes a skirmisher.
Syndra has 0 mobility outside of summoners, even though she is broken as fk right now.
Wukong doesnt really have 100-0 burst if not building glass/being ahead, hes just a tanky initiator/diver most of the time.
Just because someone has high burst damage doesnt mean they are an assassin, assassins have that AND mobility/tools to slip past/ignore the frontline (like stealth and/or high range blinks/dashes), kill the target and possibly get away alive (or kill everyone else).