I'm an electronic engineer that has worked in recording studios. I've worked on hundreds of professional reel-to-reel tape machines as well as digital audio recording systems - lining them up as well as repairing them. I also worked on hundreds of cassette decks (most studios had a couple of professional cassette decks and we also had rooms of domestic cassette decks for spinning out limited runs of duplicates).
If I was to setup a professional reel-to-reel deck I would expect it to record and playback accurately for months. This is not the case with cassettes. If you line up a deck accurately then take the same cassette out and play it again, the results will be different! This is because: a) the tape is too narrow; b) the cassette makes it almost impossible to reliably guide the tape evenly over the heads; c) the pinch roller is too small (but you can't fit a bigger one in the hole. The result is a great deal of speed variation, "wow and flutter".
Audio wise you can almost get a cassette to record and play OK, but only for a short while after setup. Audio wise: 1) the speed is too slow resulting in too much noise; 2) the tape is too narrow causing limitations in the bass (separation) 3) the top end is limited due to head and bias constraints; 4) single record/playback heads restrain the ability to check recordings; 5) split record/playback heads are very small and need to be manufactured with high precision (rarely achieved in practice). Also, of course each play results in oxide shed, so the tape quality declines with every play. Just about every domestic cassette player I've ever encountered leaves the factory with something wrong in it's alignment.
All of these limitations are immediately audible. I could record sound onto a studio-quality reel-to-reel and struggle to hear the difference between what comes out of the microphone and what's comes back off the tape. This is not the case with cassettes, the noise and dynamic range limitations are obvious as are the unreliable speed and limited frequency response (and that's on a perfectly lined-up device)
So, of all the popular domestic sources, cassette is the worst. And it's bad in so many ways.
