I mean, the mod just adds spells, so if you feel any of them is unbalanced, you don't need to use that spell.

Mystra Spells is just 5e Spells but slightly homebrewed but for the most part, keep to 5e. The homebrewing is just to make them fit into BG3 itself better.

considering the base game is horribly unbalanced its not like the mod spells can do anything worse.

Mystra's is I would guesstimate about 80% spells taken straight out of 5E source material, and about 20% homebrew. They do offer the option to separate out the homebrew stuff if you want. But then the majority of the spells are only as "unbalanced" or "OP" (terms I view as subjective, as I've said, I use them myself, but just like saying "what's pretty" is subjective opinion, so is saying "what's unbalanced" ... anyway, I digress) ... as the existing 5E spells in the source material.

There is a huge overlap between Mystra's and an earlier Nexus only mod called 5E spells. That one is purely taken from 5E sources, but has fewer spells overall, though unlike Mystra's, it includes a small number of 7th-9th level spells, and you can get scrolls for it (although it requires a separate 5E secret scrolls mod) ... neither of those things are true of Mystra's.

You can't use both at the same time ... well, you could, but all I can tell you is you'll have a bizarre jumble of duplicate spells with different icons and sometimes missing spells and icons ... they overlap too much to really be usable at the same time. Given that I have to choose, I choose Mystra's, also for the secondary reason that it not only has more spells (and the additional spells are more than just the homebrew ones, they have more spells from the source material) than 5E spells, (for levels 1-6 and cantrips) but it also better integrates with all the class and subclass mods at mod.io.

You really can't - or at least shouldn't - be playing many classes like Artificer without Mystra's spells, because many of the spells it's supposed to get as class spells as it levels up are only IN Mystra's spells.