Undecidability means loading your configuration could never complete or could consume an indeterminate amount of resources. Which means it's a security concern as a bad config can DOS your system.

This type of attack exists even without Turing-complete config but just with references: https://en.wikipedia.org/wiki/Billion_laughs_attack

> Also, we could change Turing completeness into something less powerful, for instance programs without loops.

Yes, this is a strategy. For example BPF does not support loops.