At it's basic definition, sweet feed is a feed that has been sweetened with molasses to improve its palatability. Most sweet feeds (that I've seen at least) also have high concentrations of corn and other grains, which make it very high in sugar/starch (NSC).
I don't feed sweet feed for several reasons. Besides being high NSC, the feeding directions are usually for a very large amount (6+ lbs per day). Horses' stomachs aren't designed for large, infrequent meals or high NSC feeds, both of which are known to cause stomach ulcers, hindgut acidosis, and colonic ulcers. And then, because it's fed in such a large quantity you end up hauling around a ton of bags (a single horse can go through a 50 lb bag in a week easily). It's often more expensive than feeding a higher quality pelleted grain, as well; even though it's cheaper per bag, you will likely have to buy more bags in the same time period.
I'll admit I haven't spent a lot of time comparing sweet feed tags, but they always seem to have only the bare minimum nutrients listed. Will this sweet feed supply enough lysine for a horse's diet?
Unless you get it analyzed yourself, you have no way of knowing. There are some exceptions (I know Triple Crown makes sweet feed formulations
and has a more thorough guaranteed analysis) but I think that's the exception, rather than the rule. Many sweet feeds don't offer a lot of nutrition- they're kind of like living off fast food. You might do OK and you might even be at an OK weight, but you could certainly be healthier.