Turns out there's actually a desktop outliner out there that's worth a damn, and I mean one mere mortals can use for a change. It's called TreeLine, and it does a lot of things right.
For one thing, TreeLine saves its data into honest files, wherever you tell it to save, not in some hidden database like most competition. And those files are JSON, so if you ever find yourself unable to run the software, it's still easy to recover the data, with a text editor if nothing else.
Second, TreeLine isn't just any outliner, but a structured data editor. By default, each node only gets a one-line name field. For anything else, you have to define more. You can add any number of node types to a document, each having multiple fields, and fields have various types, with suitable input methods (such as a month view for dates). Not that you have to use the outlining features: it can work Rolodex-style just as well.
Third, import and export features are outstanding: a surprising variety of formats are supported, and handled smartly at that. CSV works especially well. Also browser bookmarks. Only HTML export is terrible, but it should be easy to write filters, using the generic XML support as an intermediary. Documents can even be printed out (e.g. to a PDF file), with flexible output formatting.
Oh, there's a bit of a learning curve, except reasonable for once. TreeLine doesn't overwhelm; with it, I feel in control. TreeLine can be mastered.
If you want to give it a whirl, TreeLine runs on Windows and Linux (probably BSD and Mac, too), and it's light on system resources. A keeper all in all. Enjoy, and may your data stay well organized. We need all the help we can get.