I think that you missed a very important word in the title and URL. Expand your question with context, and it becomes its own answer:

> When would you use a standalone program for NetBSD that is written in C, over a Linux kernel module with a GPL licence, or a standalone program written in one of two languages neither of which comes in NetBSD base, or a perpetually unfinished kernel module that isn't being worked on that is for the wrong BSD anyway?

TBF, there is an experimental wireguard implementation in NetBSD as well.