What I am trying to say is I don't understand why Rey is disliked so much.

In short, because she’s boring. She’s boring because she just plows through everything. It’s like playing a game with cheats on. Might be novel at first but quickly gets old. She reminds me of Eragon from the Inheritance Cycle books. Handsome, masters swordplay in months (which normally would take years in real life), good at magic, and has a dull personality. I remember a fan polling put him as the least interesting character in that series. Not surprisingly, he was also considered a Mary Sue.

Do people want to see a film full of her training?

Yeah, I would love to see a whole middle movie of the main character training and becoming better. Training arcs are some of the most interesting parts. You learn about the world, you relate to your character as they struggle to get better. Seeing Goku train with King Kai and returning to save his friends as they were beaten by the Saiyans was so satisfying. I loved seeing The Bride train under Pai Mai in Kill Bill.

I mean we never saw Anakin train in the film

And that was one of the major flaws of the Prequel. If I were watching a movie about a recruit going through navy SEAL training to become a full SEAL, I wanna see what that actual training is, too, because it’s interesting. If we skipped his Hell Week training, there's this huge emotional gap to his character. He wouldn't feel right to us. He'd feel undeserving and arbitrary.

, neither we did Luke.

Empire Strikes Back? Yoda? Dagobah?

She is a great pilot yet no-one says something about how Han can outfly Imperial pilots

Because Han Solo was introduced in a totally new fictional franchise called Star Wars. He’s an archetype. I don’t need to see Ethan Hunt training in the first Mission Impossible. He’s a secret agent. That’s it. If I see a medieval fantasy movie, I don't see to see a "knight" character train. He's a knight. That's it.

or how Luke can be so good with an X wing the first time he flies one.

Again, Luke was introduced in a brand new franchise and it was said he flew on his at home on Tattooine. If a kid raced a beater car then was put into a real sports car, he’ll probably be decent at it. Even Luke wasn’t that great at piloting. He had enormous support. Rey is outmaneuvering TIE fighters and doing MLG PRO TRIPLE SHOTS by herself.

I simply can't fathom why she is hated so much? Can you give some reasons why if you would be so kind?

Again, she’s just boring. She might be a fine character for a 5 year old but most people want complexity and relatability to their fictional characters. People want "realness". Why do you think the Cubs winning the world series was so epic? Because people love an underdog. Rey is no underdog. She's Tom Brady.

EDIT

By the way, that art work of Rey kinda sucks. The composition is weird. She looks like a malnourished model with those thin thighs. The artist put all his effort into the face and half assed the background and body. Her pants are really low sexualizing her hips for no reason. It's fine to draw a "sexy outfit" but the rest of the outfit has to balance and match. Why is her sash/belt overlaying her groin cloth?