People will generally say that John Brown was a mad fanatic. A fanatic for a good cause, but crazy nonetheless. Generally the evidence proffered is his raid on Harper's Ferry, which does indeed seem to have had little chance of success -- it through down the gauntlet in a rather overt way, and it seems like it only threatened to nibble at the institution of slavery -- Harper's Ferry isn't exactly the heart of cotton country.
Once upon a time I myself thought it was ludicrous that John Brown would try to start a revolt in the mountains of Virginia, since while the terrain is favorable there are very few slaves.
Then I looked at a map showing slave distribution, and saw that Jefferson County (where Harper's Ferry is) is about 30% enslaved, while Clark County, immediately to the South, is 50% enslaved. On a map you can see a dagger of counties that are mostly slaves going down from Clarke County all the way to the North Carolina line. - Faquier, Culpepper, Albemarle (Jefferson's home) and on down, to the James River and beyond. So Harper's Ferry is actually fairly close to the sort of marjority-slave counties that would be good candidates for Rebellion.
Meanwhile, Harper's Ferry is less than 50 miles (over rough terrain, where concealment is relatively easy) from the Mason-Dixon line.
If you wanted to start a slave insurgency and funnel slaves to the North (and receive reinforcements from there) this would be the place to do it.