As a Laravel developer, I haven't had much of a need for a serverless platform like Vercel. Recently though, I've been working on an API that provides VS Code compatible syntax highlighting and Vercel was the perfect choice for hosting it: fast, secure, and reliable.
You can deploy your Vercel app in a couple different ways. One is to deploy from your local machine using the Vercel CLI, by just calling vercel --prod. This makes your app live on the internet in less than a minute, which is kind of insane.
Both of these options are extremely quick, but make me nervous. I didn't have any confidence that I wasn't deploying a broken build.
This means that my tests would actually run against my API live on Vercel. It's wonderful to test everything, but it gives me so much more confidence to know that after everything is deployed, it's all still going to work.
It may seem silly, but there is a wide chasm between a local machine and a fully deployed API on Vercel. There is a lot that could go wrong!