As I only worked for startups, I often was one of the few tech persons in the team. I had to learn the whole stack in order to be flexible, and Ruby on Rails made it a breaze.
Thought I’m a generalist, I have put over the years more focus on frontend technologies (React & Ember).
Keep it stupid simple unless you’re a fan of technical debt.
It allows a modular approach and a lot of flexibility.
Make sure what is built has a single responsability.
Make sure every feature you build is deemed necessary.
Build – measure – Learn. Always start with a good MVP.
If you can make it open source, you should.