I guess, from what I've seen, SHUEISHA probably have better filter when choosing to publish someone, their editor probably are experienced enough to guide the mangaka.
As much as I read webtoon, it seems like they publish a lot of junk?
There's few good and entertaining ones, but the barrier seems to be lower, like WAAYY lower than SHUEISHA.
I haven't explored Lezhin yet, but their contest sounds like being through some sort filter too