In my experience, after 1st time recognition of a specific USB mouse or keyboard, Windows will simply notify you when you plug them in and out, and both will start to work 1 second after you plug them. So, I am not worried about that.

But, I was actually assuming that the purpose of such switch is precisely to trick the OS in thinking that the mouse/keyboard are there but not issuing any command, while you are in fact using them on another PC.

My concern is more that mouse/keyboard may simply not be properly detected at startup for some reason, or that they may require some plug out/in, or that the PCs may have issues at boot, as described above, or that the hub (KVM itself, in this case) may need to be power cycled.
Those are precisely the symptoms I had in the past when using USB hubs.

Again, it was a long time ago (but I see some is still showing the same issues), and hopefully these KVM may not be suffering from such glitches. I mean, they are being used in environments where one switches from one pc to another tens of times (or more) a day, after all...

Still, a direct pass-through connection to TWO motherboard ports would make me feel more comfortable.