Every "new toy" has a learning curve
I develop lots of modules for WHMCS, also for other billing systems like WHMAutoPilot, AWBS, ClienyExec and of course Blesta and I can say that blesta is the most complet, open and easy platform for us developers, to make use oglf our imagination and build extensions for it
I also motice for the lack of info on docs, but a good developer always find a way to learn and adapt
On WHMCS you can easily create modules but you are very limited on what you can do, you only can do what they allow to do, in blesta you can do everything
Like you said, more documentation for devs are welcome, but i prefer more core functionality like fixing some issues and add new features.
This community help each outher, so if you have any quaestions just ask