When using the full USB hid input stuff on my logitech cordless keyboard, the key giving ' and * on a finnish keymap (\ and | in US I believe) key doesn't work, but switches consoles instead. Debugging the situation with evbug finds that when the keyboard is in the PS/2 port, the correct keysym 43 is given, but it changes to 84 in HID mode for some reason... Fixable in userspace, sure, but unless there's a good reason for the key codes change...
Unfortunately there is no way to make sure all extra keys generate the same keycode on PS/2 as they do on USB. It's because PS/2 and USB use a completely different method of reporting keys as well as a completely different set of scancodes.