How do we know in advance if something it going to be hard enough?

For example I have a bunch of closed PRs on my Vibe-Prolog project that took multiple attempts to get right: https://github.com/nlothian/Vibe-Prolog/pulls?q=is%3Apr+is%3...

As a specific example: https://github.com/nlothian/Vibe-Prolog/pull/214 which implements https://github.com/nlothian/Vibe-Prolog/issues/204

I'd be very interested if it is eligible!