No nested categories. They're all flat. Miniflux is ultra minimalist and "opinionated," which I ended up preferring to all the others. YMMV! If you're looking for a ton of features or customization, look elsewhere.
It's also wicked fast, resource-light, and the code is really easy to grok if you want to hack in anything. But, the update rate is a single global setting[1]. I wouldn't be surprised if it could handle far more than 700 feeds... you'll hit bottlenecks from bandwidth or the DB before anything with the app.