As others have said, an external frame pack is not the best choice for scrambling when balance is an issue. Also, in heavy bushwhacking, you will curse a lot more if using an external frame pack than you otherwise might. Apart from that, external frame packs typically carry big loads better than internal frame packs. It's obvious why climbers prefer internal frame packs, but the fact that external frames have fallen out of favor with backpackers has a lot more to do with marketing than with function.

Yep.