PPM's are here for a reason ofc but they aren't THAT important imo. For instance Dybala in my save became golden ball winner 3 times in a row and he still hadn't unlearned "comes deep to get ball" PPM. So yeah i would try to use all PPM's suggested by TFF and not worry too much when a player is unable to learn/unlearn something. Just check his overall rating and goals/assists and that should give you the right idea to keep him despite his PPM or sell him because it's making him perform below expectation.