Also consider the social effect. To the ordinary eye, it's not obvious that this blur is resistant to reversal, unlike standard blur effects. However, someone seeing the use of this blur effect, without understanding what it is, may come to the false conclusion that a non-Signal blur is safe too.
That's a good point. I've already seen at least one other person on this post comment to a Github repo that blurs faces. It might be secure, it might not be. I wouldn't trust it by default.
With a static overlay, the method is simple enough that I can evaluate the security. With a blur, I don't know the difference between a good one and a bad one, so I can only trust the reputation of the author.
I like to be able to look at the output of an anonymizer and to be able to tell myself at a glance whether it worked.
