Well, I feel very fortunate because my late DMIL & I got along. There were a few moments where things got a bit hairy...the size of the wedding guest list stands out...but in general, she wanted what was best for all her children and children -in-law, and was very generous. I was also fortunate that DH would stand up to his mother when she started to get out of control.
I would venture the opinion that there would be fewer mother-in-law issues if there were more children who would stand up to their mothers when they misbehaved. I have said to my friends and colleagues more than once that the problem generally lies with the spouse and not the in-law. My DMIL knew that my DH & I were a united front. There were/are boundaries, and she knew not to cross them. The same is true for my own mother now, although she loves my DH, so we really don't have too much issue with anything.