I'd just like to point out that difficult/annoying/controlling women can also be the mother of the wife too. It's not only the mothers of sons who can be difficult to deal with. The reason we hear DiLs complaining about their MiL, and vice versa, is probably because most people who frequent Gransnet & Mumsnet are women. Men don't tend to bother airing their grievances online so much about their difficult/annoying MiLs, and mostly try to keep the peace with their wives, which is how it should be. Just think of all the barbed comments over the years made by male comedians about their MiLs. They weren't made for no reason.
Thankfully, I have a lovely DiL, who I get on very well with. And my husband & I count her parents as good friends/family. However, my son's MiL has, in the past, been known to be controlling/opinionated/bossy and, on occasion, poking her nose in where it's not wanted. DiL, being the sweet person she is, isn't very strong when it comes to dealing with her mother, and would rather not cause any upset. DS, not wanting to cause any rift, bites his tongue in order to keep the peace, and to keep his wife happy. And I, being a paternal GP, do my best to keep a good relationship going with all concerned. It's not been all plain sailing, and there has been the odd issue over the years, but we've dealt with things like the mature adults we are, and we all now have a mutual respect for each other.
Like tickingbird says, ... It’s just people. Some are awful, nasty, spiteful, jealous, controlling etc. Some of these people are women and therefore MIL’s and DIL’s.
Let's remember that 50% of all MiLs are the mothers of daughters, but that does not mean they are all perfect, and the other 50% who are the mothers of sons aren't all bad.