1) The Sharingan makes the user try to keep up with what it's seeing, meaning that the user suffers from over stress their body just to keep up. That means more broken bodies unless you're careful.

2) It's tied to emotion, meaning that you can activate it from will alone but god forbid you enter a highly emotionally charge argument and not activate the Sharingan by accident. See point 3 for further reason why that's bad.

3) You remember everything you've recorded from it. Not sure for how long you remember it for, it's kind of implied for Kakashi he remembers everything that his Sharingan has recorded and jesus that is nightmare fuel right there. Like every time you've killed people you end up remembering that because your Sharingan was on? That's a hard nope. No wonder the Uchiha clan were insane.

4) Apparently when it first activates you're temporarily blinded. Because that's exactly what you want when you've had a shock to the system through an emotionally fueled time, being blinded. I think it may also be implied that if you're an Sharingan user just gaining it for the first time on the battle field that you might just die instead and if you don't see point 1.

5) Apparently the Sharingan glows in the dark according to some characters (Kisame to Obito ch 507, p 12 & Obito to Madara ch 602, p 1). Which is like a really bad idea when you're hiding from someone and your eyes are giving you away. Like one way street to dead kind of way.

6) According to Itachi in his novel when you use the Sharingan you see in red. Like that is the most confusing colourblind ever and also the best joke ever of "seeing red". Like how did Kakashi juggle seeing just a world of red in one eye and a not world of red in the other?

7) While this seems obvious, but also having 1 eye that has the Sharingan is implied to be only half as good as having both eyes containing Sharingan.

8) Gaining the tomoe is slow going since people generally start with 1 tomoe or in case of genii, they get 2 tomoe to start off with.