Why is Rey too powerful? The force is the force. Some have it more than others. Rey is awesome.
The problem with this is that one still needs training and guidance to control the force. It doesn't come naturally. Rey went from thinking the Force and the Jedi were a myth, to executing a perfect mind trick on her second attempt ever. She mastered telekinesis, against a trained Jedi turned darksider, who was also pulling for the Saber (a feat which, a few days later, she couldn't manage). She bested that same Jedi turned darksider in lightsaber combat. And before anyone says she grew up fighting, I know she did. It's not that she's not a talented fighter, but against someone who can predict and counter your every move before you even make it? Nah, I'm not buying it.
In fact, the only time Rey failed at anything was with her first encounter with Kylo on Takodana, which was ultimately pointless.
She's not had to face failure. She's not had to sacrifice anything. She's not had to confront her own inner dark side.
In the OT, Luke failed as often as not. His greatest victory was the impossible shot to destroy the death star.
Empire, he failed time and time again. He should have understood then, that failure was the greatest teacher, not thirty years later when ghost Yoda had to spell it out for him.
ROTJ, he finally achieved greatness by refusing the Emperor, refusing to kill Vader, but only at the last instant when he realized he was already going down the same path as Anakin. When he saw Vaders mechanical arm, instead of severed flesh. That obviously triggered the idea that he could more machine than man, too.
His mercy, was as much for his own sake as it was for Vader.
Rey is too powerful because she hasn't had to grow to her potential. Luke did, Anakin did, Obi Wan did, Ahsoka did. Every Jedi has had to start small, with only a tiny bit of control over the force, and grow to become a Padawan, a knight, a master, and eventually a grand master.
She started off with the skills of a Knight, at the very least, having had no training, no guidance, no understanding of what the Force even is.
Not to mention she's a master pilot and mechanic, which at least has some potential explanation. At least the mechanic part. But it still seems a carbon copy of young Anakin in Ep I. Master mechanic, master pilot (despite having never flow a space craft before).
She got Woke by the Force and became super powerful overnight. Not even Anakin, with his off the charts Midi-Chlorian count, advanced in strength as quickly as Rey.
Rey is a superficial character. Poorly written.