As I see it, alchemy goes beyond generally understanding how it works.
You know, Mustang himself isn't stupid either and yet it wasn't him who came up with the flame alchemy to begin with. He had to decode the instructions to be able to use it. It's unique and so it can be assumed it's hard to pull off. It's not just the ignition, but force, range and vector too after all. My guess would be, that igniting could be easily done by Ed, but for using fire with such precision and control a manual and practice is needed.
And Ed is amazing in his own right. Not specialized, but like it's already been said, he's very creative and uses whatever the situation equips him with.
Also Mustang never uses anything else but Ka-boom, he's extremely specialized. Normal Alchemists don't really have an option, they have to be specialists, since they can't carry dozens of alchemy arrays with them
