There is only one feature of internal frame backpacks that is superior to externals...that being they are more stable on the hiker if said hiker is constantly pitching from side to side or back and forth. Visualize a XC skier, perhaps. On the other hand, a hiker is far less exposed to those conditions...not immune from, but not constantly. Only in completely off trail conditions does it really become a factor for a backpacker.
The advantages to an external frame backpack include better transfer of weight to the hips, better ventilation between pack and the wearers back which is cooler and less sweaty, better organization of gear, easier to load and work out of. Put a rain cover on an internal frame pack and you funnel loads of water off the top onto your back as you hike, while an external frame's standoff distance avoids much of that.
Many people think an internal frame pack will be lighter than an external frame model of similar size, but in reality, the opposite is more often true.
On balance the external offers more pros than cons, by far.
I'm speaking to true backpacks, designed for carrying all one's camping gear for multi-day treks. For day hikes, an internal frame pack is quite suitable. In the technical terminology, a daypack vs. a backpack.