Do you regret choosing serverless (lambda) architecture?

I don't regret it. As soon as I got my application running in it, I immediately abandoned it.

I really like the idea on paper, but it's cheap and easy to have small server instances running all the time. It's also easier to understand, debug, and scale those server instances.

I'm sure serverless solves problems for some people, but it feels like an edge case (or at least, at the moment, a lot of added complexity for marginal benefit).

No. But the applications (iot gateway and email list management) were ideal fits.