I have one neat old touring bike with sidepull brakes; it is a custom built touring bike from the early 80s. No problem whatsoever when it comes to braking with full panniers going down mountains or hills.

More than a few professional bike races were won with bikes with downtube shifters. They may not be optimal for touring (I prefer bar ends) but a lot of people have toured a lot of miles on downtube shifters without problems.