You are correct about the fact that in USB interface mode all the 24 ins and outs are available at the same time. As you mentioned, the computer doing the looping would work with uTrack24.
The other limitations which will help you decide:
1. At any given time, the uTrack24 can either be used as a USB interface, or as a player or as a recorder.
2. Direct pass through from inputs to outputs is not possible.
3. There is only one mixer in the uTrack24.
a. When playing back, the mixer gives you a stereo mix of the various tracks within a song.
b. When recording, the mixer gives you a stereo mix of the inputs.
Hope this clarifies some of the doubts.