I have personally produced estimates of 2 weeks that took 2 months, and estimates of 2 months that took 2 weeks. For years, I told my boss that implementing a certain feature would be "very hard" and basically wasn't worth it. When we actually pulled the trigger on it, it was done and deployed in a week.

I'm sure some people are significantly better than me at estimates, but I haven't met them. Estimating the unknown without serious research that borders on just doing the job is nigh impossible. And we're just an in-house dev team, so we never, ever do the same thing twice.

Estimating how long to put up another wordpress site is a lot easier than estimating a new project with new requirements and new tools. I typically find that people who think estimating is easy are just doing the same things over and over for new clients, rather than doing new things all the time for the same client/employer.

Sure, so have I. I don't think estimates are good for determining a date for a contract or anything like that. I think they provide data about the tasks when prioritizing, which is valuable. Code coverage is also a really low quality indicator of test quality, but it is still useful.