So I'm actually in the process of doing Lambda versus persistent costing right now for a new project, where it's heavily load-based and very spiky, but the work on each packet of information is actually
very lightweight. The tricky part here in AWS is not Lambda, which is pretty reasonable in general--the pitfalls I'm seeing are around data storage. DynamoDB is stealthily
very expensive, either provisioned or on-demand. Right now I'm converging on lambdas for compute and RDS/S3 for storage, but what really makes AWS shine for this use case more than anything else is SQS.
SQS is so good and that there isn't a great, easily deployed (sorry, RabbitMQ) option for exactly what it does is a real push towards AWS. (GCP Cloud PubSub is close enough, but I know AWS way better than I do GCP.) If I didn't need this, and I felt confident managing RabbitMQ as a queueing solution, I don't think AWS would be a compelling solution because $60 in Hetzner nodes, in a HA configuration, could do a lot of work.
(If this thing works I'll need an on-prem solution anyway, so I'll probably have to build that too--but that's "good problems to have".)