I don't think it's true. It's in the company's interest that people are evaluated fairly, especially if they are going to give big bonus or agressively fire low performers (like some of these tech companies do).

Don't know about Dropbox in particular, but I know other tech companies who take evaluation very seriously. The manager doesn't have the final word and there's a calibration committee that ranks employee based on various data.

It's not easy to measure performance, but not impossible if you put the resource to do so.

I think what is difficult is to align employees incentives with the company's goals. For instance, focus on impact and metrics can back fire.

For instance, in my team, I have an ambitious and hard working colleague who is playing the company's game (basically, checking all the boxes needed go get promoted, playing the evaluation's game). I'm convinced it's not in the company's interest. It builds technical debt, we now have several half-baked time consuming projects that the team can't reasonable handle, dubious metrics, experiments, and systems to produce them that require maintenance etc etc... I'm not blaming him though, that's what the company is optimizing for.