I'm wondering if the whole MIDI vs audio thing is getting missed here. The OP asked two questions..

The MIDI/USB adapters work fine (usually) for MIDI. Just understand it is only MIDI.

For recording an external instrumet (audio such as guitar, which was the second question) you'll need something more. Some USB devices handle both MIDI and audio. Some do only MIDI (like the cable flatfoot mentioned earlier for $6) and some do audio (but not MIDI).

Know what you want before purchasing.

I do not work here, but the benefits are still awesome
Make your sound your own!