Here's my opinion - the thing about a tent is it's just lumpen weight and it can get lashed anyplace on the bike it'll fit. The handlebars are an option, but why waste such a handy luggage slot on lumpen weight?
A barbag takes your high utility items - maps/camera/guidebooks/penknife/phone/notebook/whatever else, and on the bars it's all immediately to hand. No opening things, dismounting, undoing straps, reaching round, rummaging thru gear - you stop your bike, pop a press-stud open and it's all there in front of you. You want to go for a walk or grab a coffee with your stuff, push a clip and you're gone.
Personally, I've found them pretty robust off-road (ortlieb barbag + ortlieb or klickfix mount). I've never had one fail or bounce out the bracket, although you have to be careful about delicate contents (i.e. scratched glasses, burst drinks cans, liquidised fruit, mashed sarnies). And if you're fully loaded, I'm pretty sure something else will bounce off or fail first.