I think it is mostly personal preference, there is no performance difference if motors are mounted inside or outside.
However, I would argue that motors on the outside might be more prone to being knocked/scratched if your skateboard is dropped, falls over, or is rolling around in your car.
This company made a product specifically to address this issue.
However, there are defiantly some constraints that may force a builder into choosing one option over the other. It mostly comes down to the deck design & battery enclosure.
Some decks have a large drop that lowers the standing platform an inch or more. Therefore the motors may need to be positioned on the outside because they will hit the deck or enclosure whilst turning.
However, there are some decks that do have a large drop-down but are still able to accommodate motors mounted on the inside, the Evo landyachtz for example. The length of your motor mount, specifically the centre distance between wheel & motor pulley is likely to be the limiting factor. Therefore you may have to choose a gear drive as the centre distance is much less than a belt drive.
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