The IGHs and belt drives are technological improvements but have high dollar and weight costs, so they are useful for niche markets. The same is true of the NuVinci continuously variable ratio hub. They use it on many Citi Bike bikes in NYC. I guess they're durable but they're super heavy and not all that efficient. It's OK though, because being able to micro-shift is nice for me, as the 3-speed models have gears that are too widely spaced, effectively making them 2-speeds for me.