If you are using a 4 yr old ( and older) 1slot, 2slot, 4slot units they are not compatible with MacBook Pro even if you replace the Cardbus Card with ExpressCard. Because the main expansion board uses an old DEC chip, which is known not compatible with MacBook Pro.
Same thing with a PCI to PCI expansion unit. If you have a 4 yr old (and older) 7slot, 13slot and 4slot PCI units they are not compatible with MacPro even if you replace the PCIHIF68 host card with PCIExpress Host card.
To make your expansion to be compatible with either MacBook or MacPro you need to replace the main expansion board, backplane + the host card.
Other expansion units may require you to replace everything including the cable and enclosure.
To identify whether the mainboard is compatible or not you need to verify if the Bridge CHIP is Pericom.
Only Pericon chip is compatible with MacBook Pro and Mac Pro.
If the bridge chip is Intel ( Dec21150,21154, 21152), then it is not compatible with MacBook Pro and MacPro.
This is what a Pericom Bridge chip looks like