It was marketed for its sheer scale, PBR modeling, and so on, but ultimately, wasn't complete at launch or even still now.

Siberian Mayhem was its Second Encounter.

If only more time or effort was put into the framework then.

Unfortunate. I feel like in serious Sam 4 we saw them struggling with serious engine because a lot of sections were ugly so I can see them moving to unreal

Unfortunately, according to official statements, Croteam fully moved to Unreal Engine 5. Serious Sam: Siberian Mayhem was the last.

Too much production quality potential in UE5 or something, or attempting to redevelop Serious Engine to be comparable to it would have been too much work to bear.

Love that serious engine

The virgin Unreal Engine 5 versus the Chad Serious Engine 4.
(Croteam already deprecated Serious Engine 4 for most current projects in favor of UE5.)