This is older but still relevant and interesting:

How Accurate Is Strava as a Power Meter? A Retest on a Long Ride with Wind and Hills | FreeTriSpeed.com

If you plug in your strava information (rider weight, bicycle weight and so forth) to this calculator and then plug in a specific uphill segment info you will get a number very close to your strava power estimate for that segment.

Bike Calculator

It's not that the strava power estimate is accurate or inaccurate, it's that people use it for the wrong thing. It's a repeatable number than you can compare to other efforts, assuming the other variables remain the same. But in the end it's just a formula based on weight, grade and speed.