Internal gear hubs are fascinating and elegant, I grew up on them, but they have numerous drawbacks, and some critical advantages:
- Heavy.
- Complex. Try to fix one in the field or find someone to do so on a world tour in East Berzerkistan. Heck, in many major US cities, to actually repair it rather than getting the line "You need a new rear wheel." Periodic maintenance, simply periodic internal greasing is a major thing, and many manufacturers don't make things easy.
- Weather fragile; If you live someplace rain intense, you need to do a full overhaul with marine grease annually. See above.
- Planetary gears typical consume 5% energy in friction, unless operating in the one direct drive gear.
- Often utilize oiled main hub bearings instead of grease, with a large diameter so more runout, so harder to adjust for a precise fit and preload, tiny steel balls so not rated for heavy loads as with touring, poor seals (labyrinth instead of rubber lip) so leak oil and are rain sensitive.
+ Ability to shift when stopped.
+ Can use a larger, more effective chainguard.
+ For very small wheels like on a Brompton, provides an overdrive to give sufficient high gear.