???
I'm guessing you mean assassins and not burst mages.
Sure, they have good mobility. That's the only way they'd ever get to their target reliably in the first place.
Sure, a few of them have Energy or are manaless. That's hardly relevant. They only need the rotation once for a burst combo on a single squishy target. They're not going to keep spamming abilities like mages do/can.
"defensive cooldowns or passives" You mean, gapclosers. As in, the gapclosers they have to probably burn to get to their target in the first place. Which means they just burned their escape.
Quit crying.