Sweet feed can cause long term health effects, like ulcers, chronic founder/laminitis (low-grade, which often goes unnoticed), stall vices, irregular or "bad" heats/seasons in mares, overweight or underweight horses, and even mild to severe colic. No, it won't effect ALL horses, or even most, but it can be a contributing factor if any of these problems should come up during a horse's life.
Instead of sweet feed, you can feed a ration balancer, like Purina Enrich, Triple Crown's 12% or 30% Supplement, Buckeye Gro N Win, Progressive ProAdvantage, Pennfield All Phase Balancer Pellet, LMF Super Supplement, Seminole Equalizer, Poulin EQUI-PRO™ MVP, EquiScience Solutions' EQUIBALANCE, etc.
Here is a good site explaining what a ration balancer is: Ration Balancer
I, personally enjoy feeding my horses oats and chopped alfalfa hay (comes in a bag), and my feed store only carries Purina Enrich (which is NOT the best ration balancer in the world), so I feed a supplement designed for horses getting no fortified grain. Right now I'm feeding Smart Pak's Smart Vite Maintanenece Grass
. Though I like the looks of the ingrediants in Ukele's Equi-Base Grass
(it has higher nutrient values than Purina Enrish 32), so I'm thinking of trying it for the next batch. Others include Select II, Balance II, Mega-Cell, Linpro, Augment, etc.
So, for my horse's "feed", they get 1- 2qt scoop of whole oats a day and 3 lbs of chopped alfalfa a day split into two feedings, plus their supplement once a day. My "hard keeper" has kept weight on with this diet on a LOT less food than when I was feeding him all kinds of grains or pelleted feeds. My horses also get bermuda hay and some pasture.
Since switching to this all natural diet my horses are better in the arena (less ADD issues, lol), my hard keeper keeps his weight on better, they are less spooky on the trail and in new places, their feet and mane/tail are growing like CRAZY without any additional supplements, and my mare's seasons/heats are a LOT more tolerable. I will NEVER feed another sweet feed or traditional pelleted feed again!!