we're a small start-up, so I worry about shipping as fluidly as possible, and being confident that my infra just runs (security, patch management, scalability, uptime, etc.) All of which serverless gives me much more easily, and at a lower cost (for now) than running my own EC2 instances.
I'll worry about vendor lock-in (or dumping serverless for that matter) once I start to get ridiculous bills from AWS, or hit performance issues, or whatnot. But I try to mitigate by relying on standard formats (Swagger) and keeping my code as close to the business logic as possible - which is fairly easy with Lambda. The only thing that is really tied to AWS is the framework we use to build and deploy the architecture: a large Cloudformation file basically. We explored Serverless (the app) to manage this, but it didn't fit our need (in particular, Serverless has apparently never heard about Swagger, so that sucks).