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.
My experience is that it can be hard to define criteria that align with company's goal (make shareholder happy).
If your boss cares about the customer, then you caring about the customer helps, but often it doesn’t matter much as everyone makes it out to be. On top of that there are way more direct avenues that have a bigger impact on your job development than whatever a user thinks.
I also recommend Gervais principle as mandatory read for young managers who don’t yet see the game for what it is.