[nybble41]

Perhaps, but so what? The existence of certain dangerous modifications does not imply that all modifications would be dangerous. If you mess with the ABS logic (or anything else) and the modifications contribute to an accident then you're obviously going to be liable for that. On the other hand you could simply install a custom infotainment unit or some third-party or DIY heated seats, in which case I see no reason to preemptively ban the vehicle without clear evidence that the modifications pose a threat to other users of the road.

A reasonable compromise would be to require any modifications to be disclosed at registration, but with a policy of approving them by default unless there is a good, evidence-based reason to think that they pose a significant risk.