What is assassin evade rate when it comes to Crit Shot, and does it need tweaking?

I feel like the majority of the time, if I try to crit shot an assassin who is running towards me stealthed, they evade it - then of course what usually happens is I eat a perf. It has happened so much at this point that I am wondering if this is WAI. If I CS or regular shot at an unstealthed assassin, it usually does the job like any other. But if I try to CS a stealthed assassin, they are always evading - and that seems broken to me. It's not like I have any chance to evade a perf. And I basically am getting punished for getting the jump on another player. Would love to get some thoughts on this. Thanks!