What other sorts of trouble can be caused by someone who has managed to break out of UML, but has not managed to break the chroot?
DOS attacks are the only things that come to mind. A fork bomb could certainly be set off in there, and the rest of the system would probably notice it.
The disk space of the filesystem housing the jail can be attacked by extending the filesystem file.
A file quota can limit the amount of disk space that can be consumed. Per-user process limits would shut down a fork bomb, although this might be detectable from inside UML. The reason is that UML creates a host process for each UML thread. If a low per-user process limit is imposed, then processes would start mysteriously failing to be created inside UML. Worse, if multiple honeypots were in use and all shared a uid, then if one honeypot caused the process limit to be hit, then all the honeypots would start failing to create processes.