With a hub gear you momentarily have to stop pedalling to change which can be a pain on a steep incline where you can lose momentum in that moment BUT the big plus is you can change to any gear when at the lights, so you don't get that 'oops I'm in the wrong gear - let's take off like a tortoise' problem. They are a lot more bombproof too - many Sturmey 3-speed hubs from the 1930s, 40s and fifties are still happily on the road doing serious miles - not something you often hear about with similar aged derailleurs...