Sessile wanderer

One thing that should be kept in mind is that maps of Roshar in past that we were given may not be actually correct. It is reasonable to assume that pre-Recreance large swats of land were under control of Singers. As such there could've been if not several nations then several nations worth of them all united under single goal: takeover of continent. I would also point out that we still know little of forms of power, their abilities, their number, and numbers of spren that facilitated them. Only clue to their numbers would be Battle of Narak where there were several thousands of stormspren alone. If we assume that average warform Singer was somewhat stronger than average human soldier and that average Regal was roughly on par if not slightly above average squire than I could see how they would pose danger to human nations.