I'm not very knowledgeable about the nutrition side of things, but I have had my mare on a ration balancer (Enrich 32 by Purina) for about ten months and I can honestly say that I've seen nothing but good results.
My mare is pretty hot by nature and somehow (maybe she's missing a few minerals in her diet/maybe she really needs the ration balancer to help her process sugar, I don't know) when she's on the RB she's much less hot than she is off it. That's probably not the same experience of nearly everyone else, but for me that's really great.
She also used to fluctuate with her weight, a lot. Like she'd go from being 200lbs overweight to 50lbs underweight in the span of a few months. It was ridiculous. Now, she stays at a steady weight most of the time. She is a very easy keeper so the spring grass caused her to end up about 200 overweight but it was quick to come off with increased exercise and after it was off I was able to keep her at a good 950lbs instead of hopping down to 850 first. I'm not sure how much of that was due to better management (I used to board her and for the last year she's been at my house so I've been able to manage her intake a lot better) but I'm sure part of it was the ration balancer doing it's balancing thing. haha
Fabio - 13 year old Arabian/Lipizzan gelding
Rest peacefully, Lacey.