One of the main challenges of building on Serverless platforms is rate-limiting.
There's nothing stopping a script-kiddie from thundering away at the Serverless endpoint resulting in an unexpected and quite high billables [0].
As for Workers specifically, Cloudflare's rate-limiting plan makes the whole thing 10x expensive at $10 for 2,000,000 good-requests [1] + $1 for 2,000,000 Workers requests. Other cloud providers I don't think fare any different.