danger is about statistics; and a note on Puerto Montt
Your recommendation of sticking to back roads reminds me that the greatest threat to the safety of a touring cyclist is not bandits or snakes, but vehicles. (assuming you stay away from wars). The easiest way to not be hurt or killed is to stay off busy roads as much as possible. Luckily, a bicycle is portable enough to hitchike with, so if you feel threatened, just stop and catch a ride. (or catch a bus or train).
As for personal recommendations, I once rode from Puerto Montt, Chile, to Peninsula Valdes, Argentina, via Bariloche, Esquel, Paso del Sapo, and Trelew. Bandit-free. No snakes, although I did see a fine specimen of tarantula crossing the road. Best of all, there's a stretch of road in the Andes that lies between two lakes that are crossed only by ferries that don't carry cars. Besides the shuttle that takes passengers from one lake to the next twice a day, there is absolutely no traffic on the road, which takes you over the crest of the Andes. It's a gravel road, and the weather's lousy (usually raining), but it's dramatic - and safe.
In Argentina, the coastal road was ridiculously windy. In fact, so was most of the Argentine desert of Patagonia. I don't recommend it for a cycle tourist, unless you thrill to dust devils.