This is a theory that I didn’t come up with, it was introduced to me by a friend, and I thought it was super interesting.
So the theory is that all alicorn magic is evil. Like it’s just straight up dark magic — nasty corrupting stuff. That’s not to say that alicorns are evil, but just that their specific brand of magic is dangerous and corrupting.

There’s a couple of things that give this theory credibility.

Firstly, both Celestia and Twilight use this freaking evil looking dark magic at various points in the show. Celestia uses it during The Crystal Empire, and Twilight uses it during Princess Twilight Sparkle. What is interesting, however, is that when Twilight uses it in Princess Twilight Sparkle, Zecora specifically refers to it as “Alicorn Magic“.

The second thing is the Alicorn Amulet. This magical artifact apparently corrupts its owner and allows them to use super powerful magic. Possibly because it contains and allows use of alicorn magic by non-alicorns? Its name and the alicorn motif seem to suggest that, after all.

Sombra also uses magic which is similar in appearance, meaning his horn could be another alicorn-created artifact. It’s the same colours as the amulet.

Because of these I think that alicorn magic is actually really dangerous to use. It could explain why the princesses are reluctant to get involved with major conflicts in Equestria, and could also explain the whole Nightmare Moon thing.

Alicorns, being a combination of the three major pony species, are able to use unicorn magic too. So my thinking is that in their day to day lives an alicorn uses its unicorn magic, which is much safer to use. The alicorn magic would only be used in very dire circumstances, as it risks exposing the caster to the corruption.

Obviously this will never actually be canon, but I think it’s pretty cool to consider