If you can tell me what's severely op, or could be issues with multiplayer (heavy lag, pvp stuff, ect ect.) that'd be great as i don't have a lot of time to go through every youtube video to see how it is
Towards the mid to late game, you can make spells that can call down lighting strikes (up to like 7 all at once, or a bunch of single ones in very very quick succession), spells that break blocks, including in a radius (if you so desire), that completely negates the use of pickaxes (essentially), spawn explosions of noteworthy size, you can add the ability to teleport foward with a modifiable range that can get you up to like 10 blocks of teleportation give or take with the right loadout instantly and with very little power cost, plus it gives you access to the wither debuff that you can add to the sword in the mod (or any of your spells for that matter). Thats just a few examples of the things you can do.
Basically my point is, if you know what you're doing (and you have the endgame gear), it'll be very powerful.
All of the spells are limited by the stats of the thing you use to cast them, and there's an upper limit on the power you can put into them, so it wouldn't get hilariously OP, but it is seriously worth mentioning all of this before considering it.
Despite all of that, it is a really cool mod, and I'd recommend it anyways because it takes a while to get the materials to actually make the endgame stuff (like several diamonds worth) and it offers a unique play-style and mechanic for smart people to make some pretty dang amazing spells to mess around with.