My impression is that the chief disadvantage of a longer crank arm is the unnecessarily wide range of movement of the thigh and both hip and knee joints (which may in turn be bad for those joints). When I ride long crank arms, my butt gets very sore. You may argue that this is only because I'm not accustomed to the longer arms; and you may be right.
+1
Originally Posted by Sixty Fiver
I never subscribed to gain ratios
A longer crank allows you to apply more torque. If you disagree with this, then find a mechanic and ask him to demonstrate the use of a breaker bar. If you still disagree with this, take it up with Issac Newton.
The reality of gain ratios are moot for most people since it is not something you can specify unless you're doing a custom build. Even if you are able to, you should be specifying crank length based on what's the most comfortable for the length of your legs. Whatever gains or losses from different crank length can be compensated for by gearing selection.