Comment by layer8

Non-Turing-completeness doesn’t buy you that much, because you can still easily multiply runtime such that it wouldn’t terminate within your lifetime. With just map you can effectively build the cross product of a list with itself. Do that in an n-times nested expression (or nested, non-recursive function calls), and for a list of length k the result is a list of length kⁿ. And with reduce you could then concatenate a string with itself those kⁿ times, resulting in a string (and likely runtime and memory usage) of length 2^kⁿ.

If you want to limit the runtime, you need to apply a timeout.