Crazy big energy requirements, the possible need for negative energy, the inside of the warp bubble being incinerated at Planck temperature* by Hawking radiation when superluminal, no way to get the bubble superluminal in the first place, and no way to interact with the warp bubble once it's up. And even if none of that were an issue, there's the possibility that particles building up on the warp bubble during a trip would be released at the end of the journey in a "wave cannon" beam of destruction.
[...]
* ~1.41E32°K