Yeah so... I'm retired. Wanna have some fun now?

[rajkosto]

Security through obscurity is never a good approach (weird, i was able to reverse engineer almost the entirety of MxO even though it's engine was only used for one game!).
Checking things and handling them properly on the server (one of the first rules of network gameplay is never trust the client) is required no matter what engine you use.
Logging into remote MySQL databases directly from the client (with the access credentials right there in the binaries) is not (see Super Meat Boy leaderboards, the new Final Fantasy 14 MMO, etc).

And no, Quest3D is not open source. Every engine supplies you with a SDK to extend them (and sometimes write game code in), but that's not what full source code means. Full source code means that you control and can change EVERY aspect of the program's behaviour. You cannot do that since you still have to use their precompiled QuestViewer.exe and associated binaries. And they want you to pay 3000 to 9999 eur for a single licence in order to ship your game. I just don't have that kind of money NOR do i see why i should give it to the Quest3D people, when they are focused on business presentations and medical simulations and such, not games. Plus their renderer architecture is pretty old and the capabilities in other areas (animations, sound, network, scalability) arent impressive, plus they have NO TOOLING.

As i said, i've been developing custom engine before UE4 availability was announced, and after that I've been integrating the specifics into UE4 because, as you said, why reinvent the wheel when someone else has written already excellent tooling and runtime engine ? And it costs me nothing to release a game using UE4, since it will be free.
(Why do you keep calling UE4 UT ? The original UT99 was UE1. And the new UT reboot that's being developed on UE4 isn't out yet but looks fantastic so far. UT2004 was a blast to play, also, that was UE2. )