AMD

There shouldn't be any loss of quality assuming the KVM (keyboard video mouse) or video switch supports the same standard as your other devices. For example if you buy a HDMI 2.0 switch and try to pass a HDMI 2.1 signal the switch would be the bottleneck and you would be limited to 4k60hz for example. A common issue with them is having handshake issues where the computer can't negotiate the signal between the monitor and the device. Generally the nicer ones will do a good job of this. Additionally another issue not really related the the switch but let's say you use HDMI for example will have higher signal degradation the longer the cable is so let's say you were to use 2x 15 foot passive cables with the switch in-between you might get signal dropout as the switch won't boost the signal. That is easily fixed by getting active cables or not using as long of cables or if your cables have poor shielding getting better ones.

I have used the HDMI switchers, there's zero loss in quality, though if there's something wrong with a switcher it won't cause quality degradation, it'll cause artifacts or just won't work at all.