Forget the whole Mary Sue debate - I dont care that a 19 year old orphan girl that never went to school could be an excellent mechanic, pilot, multilingual and whatever, there are impossible geniuses in many other franchises. Forget also the whole illogical worldbuilding and all other flaws the sequels had. I would argue that the MAIN reason why they are bad is because Rey is overpowered, gets handed her power on a silver platter without any training at all and therefore subverts the classical "Heroes Journey".

It has basically been established in two trilogies and countless other SW media, that even if one has enormous force potential he/she has to train to utilise these abilities to the fullest. Be it Darth Bane, Anakin, Palpatine, Yoda, Luke, Ashoka, Ezra, Kylo, Starkiller ect ect Canon or non Canon makes it clear. If one doesnt train, one can only use the force subconciously or the most basic force skills. Rey on the other hand can use high level force powers DAYS after learning about the force without ANY training. By the first movie she is equal or superior to a Padawan, by the second movie she is a Jedi Knight by the third a Jedi Master. All of this with 0 or almost 0 training. She displays feats and abilities so far above the first two trilogies that it is mind boggling. And this is why the sequels are bad. Rey uses writer/plot induced cheat codes, that make her so undeservingly overpowered that she is a failure of a main character and very hard to identify with, let alone like.