There is another way of looking at it: Pay for a percentage of the traffic, irrespective of how fast your application runs or what your budget is. If you reach your monthly budget because of one hacker news trending article, yeah, then no more traffic for you. It can be insane.
If I have autoscaling with an upper bound, I can stay within a fixed budget per month. If I reach my budget, yeah no more new servers, but whatever running keeps running. My business doesn't disappear, I get business continuity. Paying based on % of traffic sounds insane. How will I plan for business continuity if my monthly budget is reached with this kind of model? (without any "minimum xx requests" kind of crap free tier, business continuity means fixed upper bound AND continuing service, not OR)
If you don’t have spikiness in your traffic and provisioning for peak capacity is affordable and simple, then serverless is a bit less compelling.
[0] http://docs.aws.amazon.com/apigateway/latest/developerguide/...
To make the most of Lambda though you’ll want something that affords small code size (as there’s a 50MB limit), and has fast single execution latency.
Interestingly lambda seems to make async IO technologies like nodejs less compelling, because throughput capacity isn’t really relevant anymore.