I'm a longtime fan of outliners, and haven't really found the perfect match, particularly in the Linux world. Workflowy is pretty good. Why?
Its interface, like its display, is clean and simple.
It has good keyboard shortcuts for the few commands I need and use a lot: expand and contract outline levels, move outline levels up and down or left and right, and what Workflowy calls Zoom. (Other outliners call it "hoisting.") Zooming is particularly well implemented. Click on the button of an outline level, and it immediately moves to the top of the screen. You see only those items subordinate to that level.
Because it is web-based, it is platform agnostic.
Like KAMAS, one of my favorite programs ever (also an outliner, way back in the CP/M and DOS days), the program has an elegance that enables surprising depth.
The fundamental premise: build one humongous list. Just put everything you want to know into a single file. Searching is fast (and you can also add #tags to connect things).
Best of all, it's a wonderful collaborative tool. Create a list -- a checklist for a road trip for example -- then share it with someone by sending them the URL. That URL gives them access to view, or to edit, like Google Docs. But it gives them ONLY that piece of the outline.
It does have a few problems.
Some keyboard commands interfere with the operating system. On my Ubuntu system, the commands for moving a heading up or down also toggle the display of other active programs. But drag and drop still works.
No spell check or word count. I use outliners to write with, so those are significant issues.
The app on an iPad doesn't allow the "mouse hovering" that brings up some key functions (export).
Again on the iPad, there is no access to Help.
Mobile platform problems. There is an app for the iPad and iPhone. But not on the Android, and the browser experience on my phone (which apparently won't run the latest Chrome for Android) won't allow the "move outline heading" function. That seriously cripples its utility.
In general, having a program that has different functions on the web (based on platform), on the iPad, and on an Android makes for a confusing experience. So as much as I like it, Evernote remains a more consistent way to move across devices.