I've said this a few places but I'm mixed-race and I've always identified with half-elves and half-orcs. I particularly love Eberron's interpretation of half-elves, who have a distinctly unique culture. Having distinct mixed-race peoples and ethnicities are not "problematic", we have a word for them IRL: "Creole."
The way that mixed race/species characters were presented in the UA was extremely problematic, however, and it made me wonder if there was anyone who was mixed on the dev team.
Using the example of someone with black and Chinese heritage, they will often be told "oh you're not Chinese, you're just black" or "you don't act black enough, clearly you're Chinese." So when the rules say "pick one of your characters' two parents and have them 'count as' one of them", that really brings up some uncomfortable memories for people like me.
What I would've liked is for the PHB or DMG to have a side-bar for creating custom mixed-species mechanics, and then have the half-elf and half-orc as examples on how to build them, in much the same way that the Eladrin and Aasimar were given as examples on how to make a custom race in the 2014 DMG. That way, using the example of a half human, half elf character, we can then make a half human, half dwarf character, or even a half dragonborn, half tabaxi character.