Unity is charging per install (not per sold unit), so technically developers can owe Unity more money than they make.

Not necessarily. Unity says they're charging per initial install once you break $1M (they walked-back on the "every" install bit), but Unreal takes a cut of your royalties once you break $1M, so it's still hard to really compare them properly. If you're making a free to play game, your install number could be dramatically higher than what a non free-to-play game would need to break $1M, for example.

They're pretty different.

Unity is planning to charge a flat fee of $0.20 per install over the entire life of a game. A Triple-A developer can release a game for $70 and it earns ten million dollars. Assuming every customer installs the game maybe three separate times on average over their lifespan, Unity's gonna take maybe about $85,000 in total in runtime fees. If the game had been developed in Unreal, Epic would have taken $450,000.

But let's say an indie dev makes a great game in Unity, sells it for $5, and it goes viral (like Vampire Survivors). They make ten million dollars, Unity takes 20 cents per install, and assuming the same install rate, the bill comes to $1.2 million, over 14x what the AAA developer is paying. Epic would have still charged $450,000.

With the AAA example, Epic's 5% may seem steep for games that cost a lot per unit, but at least when a game stops making money, they stop charging money.

For Unity's runtime fee, though, as people buy new PCs/consoles/phones and install their library of games to them over and over, the developer keeps getting billed with no profit coming in. Effectively, the more games they have out there in the wild, the greater a financial burden a developer has. They'll be living in fear of some Reddit post sending 10,000 people in /r/gaming down a sudden nostalgia trip and wake up to a $2000 bill the next day with seemingly no explanation.

And this is to say nothing of the problematic nature of how Unity would even accurately assess the install count of a game, or differentiate paid copies from promotional or pirated copies (which I doubt they will). Or if a developer wants to bankrupt a rival developer, how they could just rent a click farm in Malaysia to install a game over and over again and rack up a bill too high to afford.