> ... functional programming and static typing make things more reliable.

> But this isn't a falsifiable claim.

Saying "this isn't falsifiable" is a wild claim. Indeed the claim "functional programming and static typing make things more reliable" is falsifiable, as long as you admit a statistical understanding. The world is messy and experiments have noise, so what would you use if not statistics? Anecdotes?: no. Purely deductive methods?: no; we should not expect any single technique to be a silver bullet.

Good studies and analyses lay out a causal model and use strong methodologies for showing that some factor has a meaningful impact on some metric of interest. I recommend this as a starting point [1]

[1]: https://yanirseroussi.com/2016/05/15/diving-deeper-into-caus...