Unity's Self-Combustion Engine
I think it would make more sense to take a cut of the revenue when a game is sold, as that is easier to calculate for the developers. I can understand why they chose installs though as that's easier to measure for them and they would need to trust developers with their revenue numbers as they don't own the distribution platform like e.g. Apple does, but honestly they could've just used the installation rate as a sanity check for the license numbers reported by developers.As a game developer I would be very wary about signing such a contract as I'm sure these metrics can be easily gamed, so anyone with enough malicious intent could probably install-bomb my game and rack up large charges for me, and in the end I would be responsible for proving that those installs were malicious (which I imagine could be quite difficult).