Caps are definitely hard, getting them right as both a feature as well as setting them as a customer is still an art. This holds true for 'flat rate' too, where your trading correct sizing vs downtime when either get code wrong or get unexcepted success. I've been involved in all these situations so I definitely emphasize.

We had daily hard caps and budgets alerts, but it's still an area we can do better.

(Disclaimer: Product Manager for Cloud Firestore)