Oats are high in fibre and well suited for the horses digestive system, as with anything they should be fed as a part of a balanced diet as alone oats are very low in calcium. As any feed they must also be fed appropriately for the amount of work your horse is in, otherwise you can cause problems like Asoturia, on top of that Oats can be quite heating.
I hope this helps, this is all I know about Oats 😊
I didn't read your question fully so here is the other half: Oats give more energy to the horse and gives them the energy to work (at least this is what I heard from the vet). It gives them a bit more "boost" than just hay. You shouldn't feed the horse too much oat but adding a bit to their hay is fine. I really don't know any disadvantages but as I said you shouldn't five the horse too many oats. I hope this helped!