These one-sided arguments come up every time there's a Lambda thread. They're useful and easily cheaper almost anywhere you have sparse compute or RAM requirements, even for heavyweight tasks like transcoding. A t3.nano meanwhile can simultaneously be overpriced ($3.50 for an idle task) and a bottleneck (30Mbit/s sustained network) in just as many places as a Lambda could be applied incorrectly.
They're also good for extremely bursty loads. I use Lambdas extensively for a radar imagery API that delivers the images for an Android live wallpaper that shows weather radar.
That particular use case can go from hardly any traffic at all to a hurricane of traffic during something like, well, a hurricane.
For consistent processing loads, I think I could save money by just running on dedicated EC2 instances, but you cannot beat the instant scale of Lambdas.
Shameless plug: https://play.google.com/store/apps/details?id=app.radarlove....