what exactly does the game fix?
I recently started this game along with Seiken Densetsu 3 (Secret of Mana 2), although both really aren't my style.
On the GitHub's readme there is a detailed list, and it's actually a long list of fixes in general so I can't really talk about it in detail. You can also tell it what to patch and what not to patch, so it's worth looking at anyway. A quick rundown though:
-Major musical fixes, including a fix to a cutscene where a character was singing nothing.
-Rarely, a recruited character would be Lv99 from the start. This happened at random.
-A castle gate wasn't a fixed item, so it could be pushed out of the way during the early part of the game.
-A fix for a few untranslated elements.
-Importing a save file from the first game was messed up unless the main character's name started with an M.
For the first game, where there isn't a readme:
-Fixes a glitch where you could bribe any enemy in the game including bosses, therefore causing you to be able to escape any battle.
-Makes a couple of items not infinite use anymore as they should be.
-Two of the earth rune spells had incomplete code and did nothing. There are two formulas for how they work.