They aren't at all horrible of course not. There is usually competitive friction - most often lead and started by the wife who is jealous and wants to feel sure she is Queen Bee.
Where there is friction it is a two way process, more often than not driven by the daughter-in-law's insistence that her DH demonstrates again and again SHE is more important than his mother ever was and by setting little 'tests' to prove it.
There is no need for it because the DH has love enough for both but many wives can't accept this and view the MIL as competition. There are SO many threads on here where this is evident. There was a ridiculous one recently where the wife was having a tantrum because the DH wanted to have dinner with his mother. It was clear as day that it wasn't about this because she obv didn't want to have dinner with them but it was more a test - a point of principle - his time and attention must be on me first. If I say no, I HAVE to win. This is just rooted in jealousy and very sad.
MIL mostly love their sons and want the best for them. So will not be predisposed towards women they perceive as treating their sons poorly - first possible point of friction. Second point of friction- is wives/gf of the son who they perceive to be trying to upset a previously loving harmonious relationship by pulling these little 'tests' to prove they as the wife/gf are more important (DH you must spend mother's day with me and my mother anhd not your mother (until they have children), then DH you must spend every single mothers day all day with me, my mother and our children and not go anywhere near yours otherwise you can't possibly love me and I will sulk - this type of thing).
It's all very very sad and totally avoidable but alot of wives are jealous of the relationship and go all out to queer the pitch and turn the son against the MIL ('sigh, do we HAVE to see your Mother again this weekend, she's such a bitch to me') which then does not endear the MIL to them.
Not all relationships are like this obvious but you can see this theme radiating out of so many complaints about MIL on here.