A ration balancer is just an alternative to fortified grains. Most ration balancers are low-grain or grain-free (which is better for your horse anyway) and are there to fill in the holes you would have from feeding just hay/grass and work very well for easy keepers or horses that need a low starch diet. Most feed brands have a ration balancer- Nutrena's is Empower Balance- so I'm not sure why your feed store wouldn't have recommended something more like SafeChoice from the new brand.
The protein levels look high percentage wise, but you feed much less than a traditional feed (like SafeChoice) so the overall amount of protein the horse gets is equivalent. For example, if you fed 1 lb of this feed at 20% protein, the horse would be getting 0.2 lbs of protein. If you feed 3 lbs of a fortified grain at 14% protein, the horse would get 0.42 lbs of protein, so even though the percentage is higher on the ration balancer, the horse is getting less protein.
I'm not familiar with Masterfeeds, and I'd be leery of feeding a ration balancer that says to feed it with oats, as that defeats what I consider the main benefits of a ration balancer (i.e. low starch and low calorie). I'd recommend using FeedXL.com to see if the horse's diets would be missing anything without the oats.