There is a fundamental timing issue involved with older cameras not designed with the radio trigger in mind. It does not affect direct connection or the IR/mounted-flash control of remote flashes, so a flash mounted to the camera directly or through an E-TTL cable, or remotely controlled by the pop-up flash or a camera-mounted flash will work at the normal sync speed. It is only in radio mode that there is the possibility of sync problems with older cameras.
The problem is the two-way nature of the communication between the radio controller. In IR (optical) slave mode, the camera/controller is not "aware" of the flashes at the other end. It is kind of obvious when nothing fires, but if you are using multiple flashes in multiple groups, the controller end is really pretty dumb. Note that with the radio system, there are only 16 available device slots (including the controller, so 15 flashes). Each of the flashes being used by the system is not just a receiver, it also sends data back to the controller. (The most obvious example of this is the "all units recycled" indication on the controller unit.) That includes an acknowledgement signal sent during the preflash event. Older cameras do not allow sufficient time for all possible ACK signals to be processed by the controller between the preflash and the shutter actuation, so it is possible that the flashes could fire after the second curtain begins to move.
The reason that you get a warning rather than a complete disabling of higher flash speeds is that if you are using a small number of flashes relatively close to the camera (so there are few return signals to process, and those signals will have a high signal-to-noise ratio), you may find that normal sync speeds, or a 1/3 or 2/3 stop down from normal sync, are possible. Canon is simply unwilling to guarantee that the sync will occur normally at full sync speed with older camera bodies not designed to allow for the return signals.
If you are using more than a couple of flashes at any significant distance (say more than 10m), there is a strong possibility that the flash trigger signal will not be sent before the second curtain starts to move unless the shutter is set to a slower speed than the normal sync. There are too many return signals to process, and they may be more difficult to decode. You will have to test that for yourself. Canon is willing to state categorically that it's unlikely that the flash trigger will occur simultaneously with the movement of the first curtain, which is why HSS is disabled on incompatible bodies.