The only time a hub should get hot is if there's too much friction. I could see a hub getting warm if you were going down hill for a while, but that's the only reason it should warm up, aside from direct sunlight. Of course, that's a healthy hub. Throw some grit and gunk into the bearings to create some friction, and they might get warmer, but never so hot as to ignite motor oil. The grease used in hubs is a low temp, light grease anyway. Nothing meant for serious friction.
Refs: Tito24, RiverSideRider, Stussy, Nettyspaghetti, Daniel24, Freddydapice, BrokenBMX, SuperstarDK, HardBMX_Tim. There's more, but I'm too lazy to look them up, haha.