This is incorrect. The magic user kits are generally fine and if the mages just stick to damage spells they are probably mostly in line with other classes for damage (worse single target, better aoe). But there are a lot of spells that are fairly game breaking when you move outside of damaging ones.

Eh, I’m experienced as fuck, and once you learn Simulacrum it’s basically over.

Also, forcing the DM to “deal with” you is power; narrative power. An “OP” fighter requires you to add another enemy, or increase some HP, or higher athletics DCs. The types of things they can do barely change. As a wizard levels up, you have to account for their powers in more and more scenarios until your entire prep time amounts to “how can I make this interesting and not just something the wizard negates?”. Every midboss needs to either know counterspell or have an abjurer on staff. Anywhere you don’t want the PCs to go has to be covered in guards and wards and forbiddance. Every fight needs legendary saves and the ability to reshape the battlefield to become meaningful.

The fact that with enough effort you can negate their power isn’t proof they’re not powerful, it’s proof they are.