I've had spikes of up to 300k visitors to my Firebase blog in a month, and the bill was like $150. It seems hard to get bitten by huge surprise costs from a blog on Firebase static hosting, even if someone's using a botnet to try to drive up your bill.
Edit: Thinking about it a little more, I guess you could find the largest resource on a blog and direct your botnet to download it nonstop repeatedly, and that would be orders of magnitude more expensive than even large organic traffic.
No, I was referring to your bill, when your resource usage spikes (really spikes, as in sustained Gbps+ traffic spikes). Many people were taken aback by the unexpected bills.
You can set up billing alarms and use alarms as triggers for actions.
Also, you might be the main/only site administrator, in which case falling asleep might end up badly for you.
The fact that you cannot put a cap on your spending is infuriating (the official AWS answer is "we do not want to break your business" (even if I want to)).
To be fair, some complaining or crying in such cases usually gets your bill reversed.