We'll be releasing a self-hosted server and client soon that will allow you to decouple completely from our infrastructure and bring your own SSL certificates.

However, to be clear, this is intended primarily for local development use, for traffic that isn't necessarily sensitive.

It is al about trust I guess. Many people trust AWS, Microsoft, Digital Ocean, Dropbox, Apple and others (FB should be mentioned here as well) with far more then just their local test setup (which I what use ngrok for).