* Overly artsy
* Overly political. Reviewers feel the need to give it a positive review because they agree with the message, while the audience will split because they're not as homogenous politically.
* Outraged fans. Reviewers aren't typically fans of a given franchise and so won't notice if it ruins something that would irritate a fan. The Last Jedi is in this category.
* Bribery.
Doesn't happen as much anymore since they require proof of ticket purchase to review, but it still will on occasion. And it happened enough previously that they specifically had to put the proof of purchase condition in place.