They are not "simple cables," the MIDI signals have to be converted to USB signals (entirely different protocols, including the fact that MIDI is parallel and USB is serial). Some people have reported problems with the generic non-branded ones, others have found them to be fine. The M-Audio Uno has issues with some Yamaha keyboards, so I guess that's evidence that they're not all identical. You could try a cheap one from a place with good return policy, and see if it works fine for you... and if it doesn't, get a Roland or iConnectivity for $40.
They are both serial:
MIDI is a very slow serial interface 31K bits per second, USB 2 is a very fast serial interface 480Mbits / sec
I bought a cheap MIDI USB cable couple of years back. It was poor and could only handle simple Note On/Off messages. Anything else like pitch bend caused notes to drop out etc. Do the job properly with something like ICM2 - very powerful and reliable box.