I'd have to say my biggest issue with MIL is the fact I can't trust her to use common sense when it comes to my toddlers safety. She has one of those little, yappy pocket dogs and it's constatly snapping at DD1. Both XDP and I have told her over and over to keep the dog away from her but she'll do it for X amount of time until she thinks we've forgotten or something and then starts letting him around her again. It's gotten to the point where I refuse to let her take DD1 because she can't be trusted to stick to her word.
This has really gotten to me over the past week as I've been in hospital with DD2 and my family have been caring for DS and DD1 while I've been gone. MIL has offered to take her several times and I've ignored her, I have enough to worry about right now without being scared DD1 would come home with a dog bite on her