Is it dangerous to have sentient combat computers on your ships?

Included in the Synthetic Dawn DLC, The Ghost Signal during the Contingency crisis event will affect ships equipped with sentient AI combat computers.

The Ghost Signal has five levels of strength, corresponding with how many sterilization hubs that have yet to be destroyed. For every level of strength the Ghost Signal has, which starts at 5, ships will receive a 10% penalty to fire rate and tracking.

https://stellaris.paradoxwikis.com/Crisis#The_Ghost_Signal

So no, it's not dangerous per se, but there are certainly still drawbacks that you will want to mitigate.

No, ships with Sentient AI will not join the AI Rebellion if it happens in your game.