I haven’t looked too hard, but if I’ve noticed a bias on the part of the curators, it’s that their selections seem almost random. It’s like that scene in the South Park episode that makes fun of Family Guy, in which it turns out manatees are writing the Family Guy episodes by randomly choosing coloured plastic balls.
The articles that get past the Medium gatekeepers may be fine on average, but I’m certain there are all kinds of equally fine articles that aren’t selected for curation. I know that for a fact, because my articles are all the same in quality and often in subject matter, yet only some were selected while many weren’t.