My headcannon is that all the above-average unicorns get sent to Canterlot or wherever the place is where Unicorns are trained to harness and constructively apply their power, and are thus much more stable with their magic (no random uncontrollable outbursts of magic), but much more rigid in their abilities (no random bursts of great magic, or random bursts of magic outside their usual sphere of use). Sort of like languages, once you get older, you cannot learn languages as quickly or as well, and you are more rigidly stuck in whatever language you do know; but within your vocabulary and sphere, you're rather skilled. Except if small children had random bursts of speaking swahili that could destroy a house. So, most of them are taught how to avoid using their magic and prevent surges of magical power. Sort of like an internal power inhibitor for safety. Also, most of them become specialized and, with that specialization and training channeling all their energy into their chosen sphere, they cannot do things outside their normal sphere. Very few unicorns, in my Headcannon, are trained to combat large disasters, like the Ursa Minor and Major; the risk of those unicorns becoming a terrible threat themselves is too great for all but the least ambitious and most stable/trustworthy unicorns. Other unicorns are like giant power stations that are safely harnessed to do mundane things like powering everybody's houses and lightbulbs; harnessed/restrained for mundane utility. Twilight is like a bolt of lightning: sheer unharnessed power seeking a conduit.