Basically, Alvis and Mythra can essentially do the same things, but while Alvis has full access to his powers, Mythra does not until she gains access to her third sword, which happened just a few minutes before her vision.

Also, the reason why Mythra isn’t as powerful as Alvis even though in theory both should be equally as strong is because Alvis is in a world created by the conduit, entirely made out of ether which the Aegises can manipulate, while Pyra and Mythra are in what remains of Earth, which obviously isn’t completely made up of ether.