This comment is ridiculous. You've shifted this from "too many options for everything" to "any options at all". Also, how are these things framework specific? The API vs templates argument also applies for Rails. Choosing an authentication method also applies to Rails. Choosing rendered emails via SMTP vs Sendgrid - also applies to Rails. And so on. You can't mix business requirements with "tooling choices".
Rails has a templating system for generating emails (standard HTML/ERB files). If you're running an ASP.NET Web API (not MVC) the best way of doing that I've found is via RazorLight which you have to set up manually - https://github.com/toddams/RazorLight
For authentication, with Rails the standard is pretty much Devise or Omniauth (or both) - does everything for you. I've never found anything for ASP like Devise which gives you an entire registration/login system with all the required views/models/migrations in a couple commands.
> I've never found anything for ASP like Devise which gives you an entire registration/login system with all the required views/models/migrations in a couple commands.
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You know that this is built-in, right?