I also have fed nothing but alfalfa and in 40 years have never had any of the problems listed like ulcers, entroliths, etc... regardless of whether the horses were working cattle all day, nursing a foal, competing in events or my Minis driving or just idle hanging around- nor has anyone else I know.
Spring pasture has WAY more protein and very high calcium content compared to alfalfa. Also, 20% protein is pretty high for alfalfa- normal range runs from 15-18% so I dont know where they are getting their information that some is 18-20% 'at least' with some 'often higher'??
The issue that it 'overloads' kidneys is one of the biggest myths regarding feeding alfalfa.
The energy level alfalfa provides is pretty close (sometimes even under)to what is required for various horses- weanlings and growing youngsters, to performance, to broodmare, etc... The protein content at the average 15-18%, is meeting the needs of growing foals through their yearling year. Grass has about half of what is needed.
I could probably find people here who have had horses on grass diet, who spend a fortune on supplements to find the right 'balance', who have had some serious issues with feeding grass hay. Grass hay can be constipating, and it can ball up in the stomach and cause colic. It is also way too low in protein and calcium... etc..
I would be very happy if I could get a nice alfalfa/orchard grass mix, but it is not to be found here. The bermuda is awful and I use it for bedding during foaling - it is just poor nutritionally. The timothy hay here is relatively nice, but a 3 wire bale is about $24 a bale.
I think people think they need to feed alfalfa in the same amount as they do grass hay- you should be feeding much less. Also, do not feed a bunch of hot supplements, corn, etc.. with alfalfa either.
If the horse is healthy with no other issues (past founder, sugar problems, etc...) I have never seen a problem with feeding alfalfa, nor do I know anyone else that has had problems, regardless of the breed or size of horse. A vet would know what kind of diet to put a 'special needs' horse on.
Here is a good article or two on feeding and some of the 'myths' that are also out there:
http://www1.agric.gov.ab.ca/$department/deptdocs.nsf/all/hrs3243
http://www.alfagreensupreme.com/pdf/TB%20Times%20Alfalfa.pdf
http://www.alfalfahayfarm.com/Alfalfa_Horse_Feeding_Facts.html
I am sure there are just as many pros as cons to be found on the internet regarding who feeds what.......