you use the software mixer , remember these are just line level inputs
i had a FTU for a couple of years , I got rid of it a while ago because I had 3 interfaces already but I kind of regret it , would be handy to have around.
there was absolutely nothing wrong with it , I had no issues with noise or power , the sound was clean and crisp , the pre's were ok , it was very stable . and it was cheap .
it wasn't spectacular in anyway though , if you want to focus on specific things like ultra low latency or built in dsp mixer/fx or better pre amps then other products will give better results for more money .
If you need a cheap bang for the buck reliable all rounder it does well