FMWhatsApp's Call Filtering Feature: How Effective Is It?

I'm considering FMWhatsApp mainly for its call filtering feature, aiming to better manage who can contact me directly. For those who have used this feature, how effective and customizable is it? Can you really filter calls based on specific criteria, and have you encountered any issues where the filter failed to work as expected? Insights on how this feature has enhanced your call management would be very helpful.