Unity isn’t a game engine. It’s a malware-pushing advertising network which merged with a game engine to acquire a delivery channel and now wears its skin. This is fine for the bigger devs and publishers structured around platform relationships, but for everyone else? Gamemaker, Godot, Unreal, Defold… pick a flavor, they’re all on square one.
Even if they aren’t lying and have the magical ability to determine the exact number of installs, some asshole is going to create a bot net which spams unique installs for developers they don’t like, skyrocketing the number of “installs” and therefore the cost to do business.