Some are nice, some are average, some are awful, but if there are awful MILs, it's almost always mums of sons. I only know women who have issues with their MIL. I don't know a single man who does. Anecdata of course, but I am saying what I see.
It used to be a running joke some 40+ years ago that men didn't like their mother-in-law, and mother-in-law jokes were rife. I think they must have been banter/tongue-in-cheek, because every man I have ever known gets on well with their mother-in-law.
Conversely, I know loads of women who do NOT get on with them. They find them to be harsh and critical, and act like the daughter-in-law can't do a thing right. And no woman is good enough for their little Prince! I know half a dozen women whose MILs were awful to them, and they tolerated it for a few years, and then snapped. 4 have gone NC with them. So has the husband/MIL's son.
I think these MILS do (deep down) fear losing their son, as the old adage 'a daughter is a daughter for the whole of your life, a son is a son til he gets a wife' is quite prevalent with some people. A couple's life often leans towards the woman's family and parents, moreso than the man's.
JMO. But I think some mothers of sons fear losing their son, and act all weird and possessive, and hyper critical of their son's wife, (as no woman is ever good enough for her son!) and it ends up alienating the son and his wife. I have NEVER seen this happen with mums of daughters. They always get on with the son-in-law. No mother I know has ever feared losing her daughter to her daughter's husband's family and parents. And I have never known one act all possessive over her daughter(s.) I am sure it happens. I have just personally never known it to.
JMO based on the experience of many people I know. And I know not ALL mothers of sons are like this, but they are more likely to be than mothers of daughters. My mother-in-law was OK; a bit outspoken and bossy, but OK, and she seemed to be OK with her son being with me! But she died 2 years after me and DH got married. (And before we had our kids.) I don't know how she would have been as the years rolled on/after we had kids. (2 daughters!)