This app is very nice - it has a good interface, and doesn't have many of the troubles the Neo2 Ultra Menu had. That said, I did run into something weird: When I format the Neo2-Pro cart, I get 100% good! Yay! However, if I try to format twice in a row, I get about 50% good.

I have found that as long as you disconnect the USB after formatting, it will format 100% good. Apparently something isn't being reset correctly after the format. Unplugging the cart resets it so that it formats properly the next time. This is with the Neo2-Pro in the N64 Myth.
This extends to writing in general - if I try to write a game immediately after formatting, it always fails. If you unplug the USB to reset the cart, then it works.

One other bug - the game you write is always renamed. A ".pat" is added to the file name, so it no longer shows in the list of files unless you change the filter to All Files.