A good excuse for struggling with recursion is that the person is a kernel programmer. There, on a very limited stack, recursion is so dangerous that programmers develop a strong reflex to get rid of recursion. If you tell them to produce something recursive, at every line of code their mind will be screaming DANGER and trying to fix the problem.

Such people are better tested on problems involving cache lines, TLB entries, memory-mapped IO hardware, DMA, atomic instruction opcodes, and interrupts.