You’re right, i also noticed some few people using Bge going somewhat more advanced like Mark Telles or this guy does not share their hard work features or you must subscribe to their Patreon.
This really wo’nt help Upbge growing popularity and more users.
This is an issue, i had to search a lot about doing stuff in Upbge and Python and figure how it works, i would have spend ten less time if there was enough tutorials like Unity.
Also no asset store to easy share and centralize all Upbge specific code or addons is also not making things more easy.
I can’t imagine someone coming from Unity to Upbge.
Because you want harcore battle.
You would use Unity providing any tutorials, any addons and simple C#, you won’t be doing battle but making and completing your game instead, it’s a choice.
Coding all things yourself is good for hard core programmers, it’s what they like.
But it’s really bad for non tech game devs whose goal is to complete their games with available 3D engine tools and features, it’s really different.
What i think (GPL License aside), is when Upbge will be release (not beta), to grow popularity it will have to provide enough video tutorials and not let people do it the hardcore battle way.
And to attract users a current gen looking demo is needed.
Whatever the demo uses only one character like Unreal, but top notch character animations and top notch environment, buy all those models or get a 3D artist, but a tech demo that people will find great enough)
(while we know appart from only few, most of them like most Unity users will never make groundbreaking games).
( Because if you make some not so great demo or old graphics Starfox low poly demo, there is always people that find that bad and not looking like GTA or BattleFront 2 game 
)