Honestly, customer interviews. Not just reading reviews or NPS scores-sitting down (on Zoom) and asking users why they stayed or nearly churned.
I've seen teams obsess over feature requests when the real gold is in the onboarding friction they're too embarrassed to mention. One conversation revealed a whole segment was bouncing because our "simple" setup wizard confused them. Fixed that, churn dropped 18% in two months. LTV jumped because those users actually activated.
Nobody talks about it because it's not sexy or scalable-just messy, manual, and brutally honest. But it's the only thing that's ever reliably told me where the product funnel leaks.