This makes no sense.
THE ENTIRE ENGINE SOURCE is available on GitHub. For zero payment, until you actually ship a revenue generating game with it.
What you get if you pay them more money up front, is people you can call up and talk to. Or slack. Or send email. Or fax.
The reason the bugs don’t show up in demos, is that the demos are given of paths that don’t have those bugs. Whoever gives the demo, knows what doesn’t work, and avoids those parts.
There’s also the case that CONTENT IS KING and good artists can make anything look good. The procedural generation stuff looks awesome, because a small team spent a fair bit of time making and tuning the base content that the PCG instantiates. Same thing for every other feature!
Also, AAA developers have their own programmers. They can write their own code to add whatever is missing. The can back-port code from later versions to earlier versions. They know what they want, they decide to do the work, and they own their own destiny. The draw-back is that they take on a pretty significant support burden of their own, to back-port bug fixes and maintain whatever their custom work is. AAA do be like that.
Your games are not halted by bugs in Unreal Engine, your games are halted by your inability to fix or work around those bugs. This is the same for any technology you license in any field anywhere. If there’s a bug, the vendor may or may not fix it at some point (more likely if you pay them a lot of money) but shipping the product is still your responsibility, and it’s up to you to find a way through.
If you don’t know C++, I recommend you find someone who knows C++ to help you out, or learn C++ yourself, to make more progress.