[MrForbes]

Another point that you might consider is that gearboxes are not 100% efficient, and gearboxes that have a lot of reduction (like the FP or multistage planetary boxes, and worm gears especially) can have surprisingly low efficiency. If your design does not take the frictional loss of the gearbox into account, and you calculate that the motor(s) can just barely do the job, then in real life it probably won't work.