Basing the fee on number of installs instead of number of buyers is a problem, it's an unpredictable cost that developers have no control over as it also counts repeated installing of the game. Their FAQ suggests that malicious "install bombing" is something they cannot prevent automatically but instead have to resolve via their support. In other words you depend on them having good support while they profit from bad support, all while their CEO is the kind of manager who wanted Battlefield players to pay for each weapon reload.

If Unity just counted number of buyers they wouldn't have to deal with malicious installs and users of their engine would have more predictable costs. But for some reason they don't want that, and that alone is suspicious.