RateMyProfessor reviews say more about the students who write them than the professor. Most students who respond on the site either hate or love the professor. The vast neutrals don’t bother to write reviews. It’s like Tripadvisor. You’re not going to write a review unless you really liked or disliked a hotel or restaurant. Students like professors who make them feel good, not necessarily the most effective or challenging.
RateMyProfessor is irrelevant to continuance and tenure reviews anyway. For those, the in-house student surveys (usually designed by companies like the College Board) are used. Every student is supposed to fill one out, so you get a more realistic picture of how the professor is perceived by the students.