I think there are equal numbers of threads about abusive/vile parents on here as MILs, I think generally there are just a lot of unpleasant people in the world.
The problem with MIL/DIL is that you are forced to spend a lot of time with someone you are not naturally friends with, if you met them at work or whatever you could just not socialise with them but as a MIL/DIL you still have to spend a lot of time with them.
I also think some mothers can treat their sons very differentally to their daughters, particularly in the older generation. I see it a lot in my grandparents generation, they baby their adult (mid 50s!) Sons but expect their DDs to be adults.
My MIL(or DPs mum-not yet married) is a horrible woman, our relationship isn't bad exactly, we have never fought but her relationship with DP is what gets me. She's manipulative, controlling verging on emotionally abusive to him. She refuses to believe he's grown up and interferes with everything, every single decision he makes she has to be involved with or she has like a genuine toddler style tantrum, it's hard to describe but it's just so manipulative. She's so interfering with dps life but it's all about control not genuine care for him.
Growing up she wasn't a particularly great mum, she was lazy-refused to cook for them or make them lunch, made them clean up after FIL when he was sick, would control their friends, there was lots of stuff she didnt do, if it wasn't for FIL I would have said she was pretty negligent. I have never seen a woman with such an amazing ability to put themselves above their DC as much as she did, she was really awful. She treats BIL like the prodigal son and DP like an 8 yr old boy.
It makes me so upset to see her manipulating DP, he is kind and he tries really hard to please her and she uses it to try and control him. I see him break away for long periods of time and then she slowly worms her way back into his life. He stands up to her but then he gets really upset because he thinks he has hurt her but he hasn't, she just wants to control him.
She does things like go into his and now our flat and tidy up all his drawers, throw away his stuff - (including condoms and sex toys!) Uninvited, when no one is there!. She then takes a load of his washing and does it but then uses this as a stick to beat him with, like goes on about how kind and caring she is and how shit he is basically, this is all unasked. She thought he was cheating on me so hid a load of my underwear, and kept winking at him and talking with lots of subtext. She was really excited he was cheating on me, except obviously he wasn't.
She's been horrible about some of my friends who she knew as children, doesn't know them now only knew them as young DC so she is essentially being horrible (racist, disablist etc) about young children. She has also said some telling stuff about DPs SIL, how she has taken BIL from her, how BIL only thinks of SILs feelings not MILs, how he never puts MIL first etc. She treated DPs DGM like shite when she was getting ill and that hurt a lot.
She has no friends, no hobbies nothing. BIL barely speaks to her so her whole life is just trying to control DP. Sadly FIL is a wonderful, kind, genuine man and DP can't cut off MIL without cutting off FIL. I love my FIL, because he reminds me so much of my DP but I hate my MIL. I would dislike her irrespective of her being my MIL, but I hate her because I have to watch her hurting someone I love.