I don’t think the game engine has anything to do with it. The common criticism against Helldivers 2 is that there should be more enemies, biomes, weapons, missions etc.

Adding a new enemy isn’t easy work. People think it’s just dragging a new 3D model to the map and then it’s done.

First it needs to be planned. It must be conceptually different from all other enemies so people don’t complain about that it’s just a copy paste reskin. Then it must be developed, which includes code, modeling, animation and sound design - all working in tandem.

And finally it must be tested and tweaked to ensure it mechanically works with all other systems in the game, like other enemies, weapons, missions, etc. Maybe during testing they realize it’s not as fun to play as they imagined, so they have to go back to the drawing board and iterate. Each iteration can affect code, modeling, animation and sound design. However, all involved aren’t just waiting in standby for feedback from play testing. They’re currently working with 100s of other things at the same time.

And then after a month of work they realize it’s never going to mechanically work, and they have to start from the beginning with a new idea.

Then repeat all of the above until they find something that actually works. This could easily amount to 6 months of work.