Thanks! Yes, it is for runtime checking only

Then what's the advantage compared to normal assertions, which I think would be a lot more readable.

I think normally this sort of thing is useful so you can statically check stuff. If it's runtime only that seems a bit useless.

what is the performance overhead? especially on recursive calls

Let's say I did not put a focus on speed–and there will be a performance overhead ontop of the actual checks. In a production system, you should probably consider to deactivate it in performance-critical parts (evaluate=False)