Well according to people like Robin DiAngelo, of "White Fragility" fame, everyone who is white is a racist and needs to admit it. That could make friendships difficult if we preclude friendships with racists.
As for the real world, I find it difficult to imagine being friends with someone who believes, for example, that we ought to breed out undesirable genetic samples from the population. Be it based on race or disability or whatever.
But I can imagine being friends with someone who thinks differently than I do. For example, I think a lot of the antiracism movement as it exists at this moment is a form of race essentialism, and so racist in quite a deep way. But I know plenty of nice people who don't query it that much, and while sometimes I think it leads them to say things that are foolish, I don't think it really reflects on their character.
I used to know an activist who was from the 70's school of black separatism. An interesting guy, and while I think that is basically a racist view, I didn't see it as something that precluded friendship - though it might have more on his part than mine.
I did have a friend who went through a period, after going on a trip for students to Israel, of making some pretty nasty anti-Arab comments and slurs. I had known her well for some time and it was a little difficult and I had started to feel I would have to say something. In the end though I think her father had a talk with her about it and it stopped. She was a very emotional person and generally vulnerable to being swept up into causes.