C4 is a one only man project but it is around since a long time
Same for 3dgs , it is still one of the best engine thanks to its long experience
Leadwerks is another matter
It would be my favourite engine but it is extremely slow
Some people claim that it works well with modern hardware
From my experience it is not true...it is slow...full stop
The question is
Why is Leadwerks so slow vs Unity ?
Well in my company software engineers claim that they spend , lets say 30 % of their time for the project and the 70 % left for optimization and debugging
A small team should focus on simple user friendly ,rock solid and fast game engine rather than trying to compete with the big teams