How reliable is a waitlist for validating an app idea?

I've been reading about different ways to validate an app before building it, and I keep seeing landing pages + waitlists recommended.

I like the idea of testing the messaging and seeing whether people actually sign up, but I'm not sure how accurate that signal really is.

For anyone who has actually validated an app this way and then went on to build it, how accurate were the validation numbers compared with what happened after launch? Did the waitlist signups translate into real users or paying customers?