SBMM is great for all players involved, it ensures bad players are given an environment where they can improve while still being able to have fun and win. Good players on the other hand are constantly challenged and pushed to get even better.

The only people I have ever seen criticize SBMM are either ignorant of what it actually is or just want to bully bad players to feel good about themselves. Half of the time, people who are calling it bad literally end up arguing in favor of it.