Artschool dropout here. People have mixed feelings. On one hand, he taught a lot of people how to paint and (more importantly) not be afraid of paint. On the other, he didn't challenge himself and was doing the same thing over and over, creating a lot of kitsch and commercial art.
As far as I know, he never had any long-term projects or stuff done in private. This means he plateau'd and stayed at the same skill level his entire career. Compare that to people we consider masters, like Picasso or Rembrandt or Michaelangelo. They were constantly looking for new ways to challenge themselves and trying to unlearn everything they knew to create something new. We have a lot of discoveries because of it.
Picasso invented collage and expanded our knowledge of 2D design. Rembrandt created the building blocks for Impressionism and Expressionism, while uplifting printmaking into its own artistic medium. Michaelangelo went out and practically invented new colors. Bob Ross, by comparison, didn't do much other than show how to paint something quickly. And speed is something all artists get better at with practice.
Furthermore, some of my professors resented Ross for reinforcing realism as the only way of creating art, especially among normies. Ross didn't do this intentionally. It was just a side effect of making landscapes easy. Keep in mind, we're still in the Post-Modern era and a lot of my instructors were the Post-Modern rebels who suffered under their Modernist mentors. So for them, it's frustrating trying to move art forward and have people still cling to what was already done 75 years ago. Even more so when they know the Modernists before them did put a lot of labor into their work.
Using Picasso again as an example, he'd start by drawing something realistically. Then he'd draw it again, but with less detail. And again. And again. And again. He'd get an object down to having as little detail as possible while still being recognizable as what it was supposed to represent. Then he'd compose these drawing into a painting. For Guernica, his magnum opus, he spent an entire summer doing this. There's thousands of drawings for this one, single painting he did and it's a huge painting (like 10 feet by 15 or some shit).
Now imagine you're an art professor who knows this and some asshole wearing a Bob Ross shirt walks into your class and says "Picasso paints like shit."
(I was told not to wear my Bob Ross shirt again)