I compost on a large scale and because of availability horse manure is one of the main ingredients. It's really not that great on it's own but can be a good base. The C:N ratio of course is highly variable so every batch needs to be treated differently. The biggest advantage of horse poo is just simply volume and the fact that it's there. Almost any other kind of manure I use is superior, but it's never
enough. One caveat is that many horses are treated with de-wormers and I have been told that the medicine will kill earthworms. I have no way to verify what kind of levels to expect or even if this is a bunch of hooey.
Typically the horse poo I get is way overboard on nitrogen and yes, will stink like hell. I just make sure I have lots of carbon on hand when I get a new load and mix it in. You can -- actually, you really
should -- use a carbon shell on your compost piles, which will reduce moisture and nitrogen loss. I cut straw but you can use anything woody.
There have been loads that were the opposite -- too much C, so you just have to pay attention. Like most people I just go by temperature and smell and turn the pile or adjust composition accordingly. Yes, compost will compost in the winter. Depends on climate, rains, etc.
I have in the past used raw horse manure as the base for various beds or HK beds, but I would never attempt to plant in raw horse poo right away -- if you put a 6-8" topping of compost/soil with the raw horse poo underneath it works out pretty well if there is enough air in your beds. This was my method for a passive
solar greenhouse -- the composting poo gives you a nice
boost in temps.