Maybe she isn’t a complete alicorn yet. Which means other than flying and classic sorcery, she is outclassed by other earth ponies.
So what was the rainbow ray that came out of the “hole” and to the Unity Crystals? She jumped straight out of the rainbow which is a typical feature of pony magic all the way back to G1, although it is also partly meant for harmony magic. But here it’s just about pony magic - and it was Sunny who broke out the earth pony magic from the mysterious dimension that can be the elsewhere.
Until the end of MYM, Sunny has had poor control of her alicorn state, and when she was unexpectedly transformed in her carriage, it looked like it was typical earth pony magic that overturned the carriage. After MYM it is clear that she will then have better chances and from the IDW comic we know that she had learned how to fly and use telekinesis.
The alicorns are traditionally much stronger than all other ponies because they have magic from all three basic ponies - air, material, soil. But this could be multiplied many times by a normal pony in matters of strength. Sunny is thus much stronger - but she, like the G4 alicorns, has not had the opportunity to demonstrate this (an exception in IDW comics).
So if the mysterious alicorn and Sunny are going to fight each other in the near future, the ponies will discover how strong alicorns really are, because the world will tremble. The duel between Twilight and Tirek, Cozy Glow’s period as alicorn, the battle between Celestia and Nightmare Moon - they are in a completely different league, although they are no less vulnerable, in the Ponyverse even the weakest spell can be fatal to the strongest magical creature .
Then Sunny needs all of her strength in the face of a much older and stronger alicorn if she has to save the pony magic and the Pony kind from tyranny. She must be pegasus, unicorn and earth pony as a balanced being.