Very few unicorns are as powerful as Twilight and the princesses. All unicorns are capable of performing magic but only a few are very skilled at it. It takes years of education and practice to be a very powerful unicorn. The most basic trick that unicorns can do (and one of the first ones they learn as a foal) is telekinesis and most are only limited to picking up objects like pens, cups, books, small tools etc. It takes a lot more skill and magic to pick up heavier objects (like huge boulders). Spells like teleportation, phasing through solid objects, force fields, etc. are limited to the highly skilled unicorns which only make up about 30% of the unicorn population.
This became my headcanon after I have noticed many bronies boasting that unicorns are a lot better than pegasi and earth ponies as well as many people making unicorn OC’s who’s magic capabilities can compete with Princess Celestia. (It shows your OC doesn’t have to be an alicorn to be a Mary-Sue.) I also highly doubt that all or even most of the unicorns in MLP are as powerful as Twilight or Celestia. The same applies to pegasi and earth ponies with their respective abilities. Most unicorns are not as powerful as Twilight, most pegasi are not as fast or agile as Rainbow Dash, and most earth ponies are not as strong as Applejack.