"The API is extremely easy to use.."

Damien is spot on here. This is one of the nicest implemented APIs in WordPress right now. Regardless of whatever stance you take on it (because that's not the point of the this comment), working with The Customizer itself is great from a development perspective.

It's almost recipe-like. There's a certain set of steps to follow. Once done, you've got a feature implemented and it looks great. Sometimes it's as fast as a day. Other times, it takes something like a week. From there, it can be more.

Regardless of how complex your feature, The Customizer as an API is underrated.