well you can start with this very thread
and expand your search to the entire site with some googlefu
lots of unsprung weight, poor handling.
low torque (lack of gearing, otherwise 350nm isn't too shabby), might get stuck on a hill etc.
low power, comparatively. If you geared these down so you could climb a hill, you would have a top speed of like 10mph.
electrical bits splashing in puddles.
bouncing your motors off of curbs and ruts.
more rotating mass, more energy consumption.
less room for things like brakes.
motor cables flexing and work hardening, and generally being more exposed.
Virtually nobody does it, and those that do don't share all the relevant details, success is measured by if it backs out of the garage with little to no assistance.
they are fairly practical on a commuter bicycle fyi, and with various mods you can push them hard enough to actually be good. But those are simple no suspension and already lightweight machines.