Lowest rideable gear ratio
One issue is that it can be harder to balance a bike while spinning like crazy than while pedalling more sedately. Timing a gap in traffic, for example, I can get below 1km/h, but changing right down is a mistake because when I want to get going again I'm trying to push 100rpm after a couple of strokes.  This is worse if I take the MTB on road, because of its smaller small chainring - it has very limited use.

Another is that gearing too low produces too much torque, which is worse on steel uphills. With your weight on the saddle or over the back wheel, the front wheel will lift, but with your weight forwards the back will slip. I've had this on trails (MTB) and wet roads (tourer). You have to push quite hard to get up enough speed to get through the weakest part of the stroke (when the pedals are vertical) so simply starting gently isn't an option either. Because of the grip, this heavily depends on the surface, so it's not just about the bike.