Writer’s preference. Best they go with what comes naturally to them, with the understanding that the perspective they go with isn’t going to catch all of a given audience’s interest. I would advise that they push themselves on occasion - if they’re a writer who cranks out reader inserts with fair frequency (say, in the ballpark of one a week), maybe every third one, they should switch it up.

The common ailment across the board, regardless of I/you/they, is when authors make the physical descriptions way too specific. One or two superficial things (hair length or height, for instance) that are mentioned sparingly? A reader can typically let go. But if the goal is truly to put the reader in the character’s shoes, constant reminders of specific traits that have no bearing on plot is going to jerk a reader out of the story, no matter how great it may be. An exception here is when a writer is purposefully targeting an under-served element of the fanfic reader population (male protagonists, trans, persons of color, ace, plus size… I could go on; not model-bodied straight white cis, is what I’m getting at).

Personally, I don’t care for second person (excepting quickie one-shots), because it’s this constant stream of “You did this”, “You did that”, etc. My mind’s knee-jerk responses are YOU DON’T KNOW ME and YOU’RE NOT THE BOSS OF ME and NO I WOULD *NOT* DO THAT.  🤷 How I’m wired.