It sounds like sys admin Pro will be an exception since it is so tightly coupled to the core freepbx distro.
I leave it to the devs to say where most bugs exist. I know I've had a fee bugs, but I don't dive into the code fixing it to know where they originated.. EPM Examples; dual patterns stopped working, firmware slot selection stopped working, editing the basefile wouldn't save changes, I'm having an issue currently with not being able to save changes to an extension ucp custom settings from EMP just get errors in the gui (they don't pick up the default template option but instead are using ucp custom settings but those are blank for some reason), and a few in forgetting. EPM is excellent so don't take my comment the wrong way, but some of those bugs would make it near impossible to use if not addressed. System admin just doesn't always work for updates. Nothing would get the latest -6 upgrade to take +even though the GUI said it did) and I ended up on the CLI instead.
I'm just thinking of the user who submits a ticket because something is broken in a commercial module. It takes 3+ months to address the problem created by that update. Meanwhile the user runs ocer the license period since they only had 2 months. Tgey don't get the fix. That's unfortunate IMO.
I doubt we will ever see an upgrade/trade up system. Unfortunate. Someone who spend ~$600 in a bundle and after a month they realize they need call center (didn't realize they couldn't make a queue work now they wanted) isn't trying to game the system, just looking to reasonably compensate you without double paying.