I don't know what other problems Jailbreak has, since I don't know how complete it is. It was Holger's pet project, so I only know as much about it as anybody else that has played U4E. But I know it does require Bonus Pack #4 in order to use it's one, and only one, included level with U4E.
Boy, we could REALLY use some more levels for all U4E gametypes! (Fortress, Gauntlet, Jailbreak, CTF, and Ect)