I think it depends….
I try hard not to feed more than 20% alfalfa…but if it was all I had access to for a season, I sure would try to make it work. Its quite high in calcium, which is its major drawback, in my mind. That might not be an issue for a young growing horse, whose calcium requirements are higher. But for an older horse all that extra calcium can lead to inflammatory issues and arthritis.
I bought my main saddle horse right out of a alfalfa field in Colorado. He was fat and healthy looking, but he has really suffered over the years with joint problems. Whether it was the alfalfa or just him, I don’t really know…..but the joint issues have resolved with a more balanced hay ration. Just my own experience.