Assuming that the horse is out on pasture and eating grass all day, and “all-in-one” feed on top of that doesn’t sound too bad to me, but for horses that don’t get any grass like mine-who live in dirt pens-it would be expensive and unhealthy. Not only is long-stem fiber important for their digestive systems, a big slow feed net full of hay keeps them busy nearly all day.
The “all in one” feed isn’t meant to replace grass, so it’s not like “grass plus a little on top” or “only all in one” 
Hay is just dried grass, minus a few important things (Vit E, Vit E, Omega 3). You’d feed a complete feed with hay, just like you’d feed it with grass. The less forage (grass or hay) the horse can eat, the more of the complete feed you can/should feed.
