I’m not sure how many people here use Tascam US-122 and the US-122L ( I have a 122 and love it). It came to my attention during some recent kernel testing and a subsequent Google search that somewhere along the line after Kernel 3.6 there is a bug in the Kernel that breaks operation with JACK, the device will initialize as normal and will appear as expected as an ALSA Device. Starting JACK brings things to a screeching halt and the server will not connect and error out. This has been observed across various distros. So if you record with a US-122 I suggest not upgrading your Kernel past 3.6 until this gets addressed by the upstream Kernel devs.
Have a US–122L myself, and it has worked very well until recently. Didn’t work with kernel v3.7-rc1, and that’s where it started. The bug is also described here https ://bugzilla.kernel.org/show_bug.cgi?id=53241 Takashi Iwai asked if I could bisect, but I don’t know how.
For the record work is being done on bisecting this problem at the AV Linux forum by Trulan Martin (Maintainer of the AV Linux Kernels). Anyone with an affected Tascam 'US" device would be welcomed to kernel test and give their input, the more Tascam devices represented the better the chance of getting this fixed completely and to the attention of the upstream Kernel folks.