My issue isn't the automatic mode switching. I actually think that idea is quite safe.
My issue is that the keyboard mode lacks a kind of "purity".
In keyboard mode, the game retains bits of controller code, which will be imposed on devices, no matter how generic they are.
I did test with an official Xbox One controller, and that worked perfectly. However, if you use any other kind of controller, the game will stay in keyboard mode (which is fine) but D-pads and Button 1 (whatever it is) will still have moves forced on it.
It's up to you what you do with this information. I've seen many games that I would call "Xbox controller-only". I don't like the label, but when the game is high-quality, I can still be happy about that.