I created PartsBox (https://partsbox.io/) and I'm quite happy running it. It's a tool for companies building electronics (also available for free for hobbyists/makers). The business was (and is) a "freedom project" for me: I wanted to be independent of everyone, so no investors, no partners, and no employees. So far it has worked out pretty well.

The nice thing about running a business in a niche is that you get to interact with nice people. My customers are engineers, I practically never get those mythical "toxic customers".

The bad thing about running a solo-founder business is the stress and anxiety. These are difficult to deal with.