[[...]] is non-portable and has an extremely quirky corner case with variable expansion in arithmetic contexts, what's not to love?

It also does wildcards though, with POSIX you'll need a case statement for that.

I'm intrigued - any info on that?

I personally use ((...)) for arithmetic tests and [[...]] for all other tests as I just target new versions of BASH and don't care much about POSIX compatibility.