I would agree with this. As with many others, my primary experience is with FreePBX/Asterisk and I have found a lot of that knowledge to be quite transferrable. In my fairly short time testing things, there is actually a *lot* I like about FusionPBX over FreePBX (the true multi-tenant feature is probably the #1 killer, IMHO).
It is a shame that that the training is so expensive though (and the discouragement of documenting HOWTOs and such)... at this stage for myself (combination of just testing things out and also feeling I already know how to do a lot of the functionality of it), the price is a bit steep. If it was in the $295USD-$495USD range, I would probably whip my credit card out without almost a second thought and sign up.
Then comes fixing bugs, talking to people, rewriting it to make features better, and scale well, supporting people that use it and more...
Now after all this people need documentation which is yet another good sized project for something like FusionPBX. The documentation has take some time. Open source can require some patience or some support to speed up the process.
If you look at the documentation you can see there has been many improvements. You can export the documentation to a PDF which currently shows 307 pages. http://docs.fusionpbx.com/en/latest/
FusionPBX uses FreeSWITCH under the hood. You can find documentation for FreeSWITCH here: