Barley provides more digestible energy and total available nutrients than oats, but it doesn't quite reach the levels of maize. It is less likely to trigger "hot" behaviour.
It's considered a good feed for putting condition on a horse, but it's certainly not the perfect feed. It has a poor phosphorous/calcium ratio (maize and oats aren't great either, but barley is the worst of the three). It also lacks vitamins A and D. All cerials require a balancer as the extra phosphorus reduces the ability to absorb calcium.
The grains of barley are very hard and need to be crushed or rolled, or cooked before feeding. It should never be crushed too finely, and, when cooked do so without crushing or rolling. As each grain of barley locks in its nutritional value. As soon as you crush a grain, the nutritional value will begin to decline. A bag of crushed barley fed out over two weeks will have considerably less food value after a few days.
The result can be a gluggy mass of food that runs the risk of packing down in a horse's stomach and triggering a colic attack. Like many grains, it simply does not have the bulk that a forage diet mix with something to prevent bolting of feed sugar beet pulp, chopped hay or wheat bran, lucerne chaff, even rolled oats, ensuring the bulkier feed comprises 15 to 25 per cent of the mix.
Some horses dislike the taste of barley. It will generally be more palatable when cooked, or by adding molasses.
Over feeding energy simply makes some horses fat and has no effect on their temperament whereas other horses become very high-spirited. Thoroughbred types are more prone to react this way than warm-bloods or native ponies but there are plenty of exceptions.
Over feeding Barley and Maize can also cause rashes in some TB types.