Naively, I legitimately thought people were being ironic when at first I saw complaints about not being able to relate to a character after a race change. If they coupled their sentiments with racist vitriol, then I knew they weren't playing. I actually kind of did a double-take when I found out that
relating
to a character was a real thing. In all my life, I can really only think of about 3-to-5 characters for which I could say that I "identified with them."
Nationality means more to me than ethnicity. If a movie features a Black American and a White Brit, I'm going to identify more with the American despite far and away having more shared life experiences with the Brit. Sex and gender are important factors, but there's always the universality of story-telling. If the underappreciated worker gets a better job, I might relate to that situation regardless of whether or not I fall into the same demographic (race, sex, orientation, etc.).
Religion can be important. Of the few characters that I've found myself able to easily identify with, one was Muslim and one was Buddhist. If you have a bunch of characters of different religious backgrounds, it's going to be pretty impossible for me to relate to a character who is of a faith other than Christianity. I guess Iron Man is the most relatable superhero for me, despite him being an atheist (and a billionaire from birth), but that's a bit of a dummy prize.
Choice of love interest throws most things out the window. If there's a character who looks like me, talks like me, has similar interests and opinions to my own, etc., but he chooses the sardonic and bottom-heavy goth over the upbeat and top-heavy cheerleader, that boy ain't me an' ain't a-never gonna be me. Conversely, if the Arab-Muslim guy is the only one to see that the vivacious redhead is literally the perfect woman, I'll identify with him until the story makes it impossible to do so.
"So what if he's another religion and a member of another ethnic group?" "So what if he came from poverty and I didn't?" "So what if he's a vegetari-Okay, I kinda like meat too much to part with." "So what if he's a socialist and... Alright, alright. I'll admit he's not me. He has hella good taste in women though."
Though I've stepped into the trap once or twice, I'm not a huge fan of the "This character is you" trope. I'd rather have characters independently be written to be who they are than to try and force relatability. I'm not a market, I'm me. If a character ends up being relatable, more power to them. If they don't, they should be entertaining enough that I'm able to enjoy their story without the self-insert fantasy.