The problem is that he takes too long to scale, like his passive, also it deals magic damage which means part of his damage gets reduced twice, by armor and by magic resist, AD scaling champ whose burst is partially magic damage is bad.
Being an assassin that scales much slower means that by the time you have enough damage teamfights happen and you are next to useless.
Assassins want to flank, get in, get out, if he gets in flanking with E the only safety tool is his ult, what then, you have no way out, many assassins do, kha jumps away after kill or can stealth, Zed goes where he pleases, Talon can jump any wall if you plan your route right, the list is long.
The only thing he can do best is to catch easily isolated people because his roam is fast with mobi+youmuu+e.
Rhaast is much more reliable, had no problem being a pain in the ass for everyone.
Also SA form E still bugged, it ends early after you get out of the wall even if the ability has the extended duration.
The extra range on the ult is not always that useful since you want to deal damage while being close .
Ult should have higher base damage and passive turned into physical damage, true damage, or needs to be calculated pre mitigation rather than post, or make ult reset his basic abilities so he can do smooth combos such as E-W-AA-Q-R-R-W-AA-Q-E away without the need to wait in ult for cd refresh.