Probably the easiest type of speed reduction to make. They may pop off if the sprockets are not properly aligned, or if the tension is not correct. It is a good idea to use chain tensioners to take up the slack and compensate for stretch unless the sprockets are very close together. Of the 10 chains in BioHazard, the four longest have chain tensioners. This refers to timing belts, not V-belts.
Belts are wider than chains, and a multistage reducer can take up a lot of space.
They have much deeper, and more scientifically designed teeth. I used 10 chains and 24 sprockets in BioHazard. The two-stage chain reduction, and all the misc. All moving parts use ball bearings. I've got a 21,000 RPM motor.