As far as balance goes, I don't think it'll be an issue. It's a mount, it'll have a limited health pool and once you're knocked off toy need to exit combat to mount again. It'll also probably be less flexible than foot combat, doubt we'll have profession skills or features.
If anything I hope it paves the way for more mounted combat on old mounts, could be a good way to breathe some life into the mounts that don't fly.