Integrity, mainly. Elon is not an honest person and he will use any power at his disposal arbitrarily to silence people he dislikes when he feels like it.

The most absurd example of all is the very recent case where Elon pretended to be a pro gamer and got caught. A streamer called Asmongold covered the topic on his stream, which triggered Elon to arbitrarily remove Asmongold's verified checkmark and remove his gaming badge. Considering the low stakes of this matter I find the actions ridiculous and don't trust Elon with having basically admin access to the platform at all.

I've also heard plenty of horror stories about the ruthless way the engineering in X is currently done, often carelessly breaking stuff. However I have to point out that the service is and has been far far more stable than the "haters" have predicted back when Elon took over and fired all those people.