To my mind (and I might be off the mark with this set of assumptions, but...) external frames are superior when the external profile of the pack doesn't matter, and when maximum load carrying versatility is needed. Internals, on the other hand, seem better when you need either a trim and streamlined profile (mountaineering, navigating tight spaces where you don't want to get snagged, shedding large amounts of snow, etc.), and/or when you're getting into the ultralight realm, where you start minimizing pack structure (ie. trading full frames for simple framesheets with no other rigid stays) to save mass/weight.
Mostly agree although it's worth noting that UL pack makers like Vargo and Zpacks are using external designs now.
Closely in line with this, even my Kelty Tioga is lighter than a comparable expedition sized internal frame. Simply less heavy fabric and padding involved and that really adds up.