Are ratings the only thing that affects player performance, or does experience play a role as well?

One example: I have a player whose ratings were ML ready, but he had never played above AA before. He struggled in his first 20-30 games before starting to hit well. Is this because he lacked ML experience? Or was it just had luck?

Another example: hypothetically, a guy with fielding ratings of 60 across the board would make a good LF. But if his player had spent his entire career at, say, first base, would he be able to transition selves sly to left field? Or does the game build in a learning curve for players who haven't played a position before?

No.

Bad luck.

No.

Ratings are literally the only thing that matters

That said, sometimes ratings play out differently than you might initially expect. The best example is range. You will sometimes see people wonder how it is that if more range is a good thing than why does their fielder with higher range make more errors. The answer is of course that a by product of getting to more balls is the chance of punting them. A guy with 10% more range should be expected to make 10% more errors all things being equal.

Also some ratings seem to play nicely with others when they are both high. Speed and base stealing, batting eye and contact are two good examples.