I have mortality and major medical for my horse.
One thing I noticed while shopping around is that the major medical coverage is handled very differently between companies. The one I went with (through Broadstone Insurance) has a deductible (~$250 I think) for each instance, but then covers 100% Another company I looked at had no deductible, but only covered 60%.
The only time I had a horse get injured was a leased horse, who got a small puncture wound to the inside of his knee, which punctured a joint capsule. For something that looked so small, it cost a lot ($450 or so). So when I bought my own horse and was insurance shopping, I figured I'd rather know that I'm paying $250 for the deductible and then be free to follow the vet's recommendations, rather than have to decide for each little thing if I could still afford it.
That being said, I've never had to file a claim and for non-emergency situations I'm supposed to get the treatment plan approved beforehand. So only time will tell if the insurance really is as useful as I hope.