> Case studies, focus groups, surveys and interviews are great ways to find the unknown unknowns. Of course, you need to pay people to participate in them, and then you need to pay expensive employees to conduct, collect and analyze the results

No they're not, because the vast majority of people simply won't be bothered, and most people probably aren't as reliable as concrete data.

People will be bothered if you pay them. DigitalOcean does this with focus groups for developers, and offers $500+ each for an hour or two of developers' time.

some people might be bothered if you pay them. Those for whom money isn't an issue and/or are too busy won't, so that will skew the results.

I was thinking of those as thing you do before product release (so they're "known"). But it's not a good way to find out about reliability issues, because those only happen in especially weird situations, or over time like running out of disk space.

Telemetry that tells you which features are popular is useful but does need filtering to avoid identifying individual users. But sending back errors and crashes is what's really important.

You can do things like have feedback forms but typically users don't like sending that in because they feel like they're doing work for free.

You can collect data from people who opt in.