It's an interesting one. I think you're probably right OP, there are always two sides (to a certain degree). Equally though, not every MIL story on here comes from the same poster so I'm sure they all have varying degrees of justification.
My MIL from a previous relationship was fine. I now have a step-MIL and MIL. The step MIL is a truly lovely woman but my relationship with MIL is REALLY challenging (as is my husband's).
She said some really nasty things about me before I ever met/spoke to her. I have no idea why and if I'm honest it tainted my view of her before meeting her too. I was really disappointed that she didn't want to like me as I was excited about the having a relationship with the mum of the great guy I'd just met. She came to visit and tried to persuade him to split up with me citing my health (I was young and fit but had mild asthma!) and the fact that I didn't sweep up some toast crumbs from the counter immediately(?!). I found it crushing.
Only when I was pregnant did she seem to mellow a bit. But when we'd meet on Skype she made comments about my excessive size and how unhealthy my pregnancy looked (I'm size 6-8 and only put on my bump weight). We tried to put it down to her feeling out of the loop due to living so far away (North America).
She said she didn't want to visit when the baby was born as we were planning a trip to her when it was 4 months old and had vaccinations out of the way. When baby was born she then sprung on us that she wanted to come and stay for a couple of weeks in our tiny flat. Our hearts sunk but we didn't want to offend her so carefully worded a really friendly email saying we'd love to have her, how glad we were she'd changed her mind... but was there any chance she could come 2weeks after her chosen date so I could settle into the breastfeeding a little more as I felt self-conscious. She refused and told all her friends we wouldn't let her see her grandchild and that she'd never forgive us. I cried.
Fast forward two years and we have two children. She dotes on the first and openly tells me she prefers him (argh). We are so exasperated and exhausted by the situation but are getting better at managing it and making sure that we don't fall in the trap of treating her unfairly in anticipation of a nasty remark (we can't account for others' behaviour, only our own).
I really sympathise with everyone who finds MILs hard. IHowever, i's rare that we remain wholly innocent in the situation, especially after years of feeling persecuted. And I still feel sorry for my MIL. She does these things because she's insecure; she has no close and loving relationships in her life and she doesn't see that her own attitudes have caused that. It's sad.