I discovered this this past weekend during a two-day recording session on location. It can cause BSOD (Blue Screen of Death) system crashes, the DAW may not 'see' the interface when used in ASIO mode, and, worst of all, it may introduce crackles, static and other distortion into the audio output (this distortion does not appear to be recorded and is heard only when tracks are played). Specifically, the ASIO implementation is extremely unreliable when used with, at least, Reaper, Sonar and Audition versions 3.0 and CS6. However, they are also BUGGY to point of being non-functional in some contexts. The new drivers offer a different graphic interface, but no real change to functionality.
I don't use Mac, so I can't say the following applies to the Mac version of the drivers.
In August of this year, Avid released new drivers for the FTU and the FTU8R for both Windows and Mac.
Last year, m-Audio, the manufacturer of the Fast Track Ultra and Fast Track Ultra 8R, sold off its semi-pro and pro line to Avid.