This whole thing is 1) open source (under ZLIB which is incredibly open), and 2) written against the wxWidgets UI set which compiles on basically every platform (it was the cross-platform choice before QT was LGPL'd -- which I presume had something to do with the decision to use it, as TreeSheets has been around for nearly a decade).

I for one applaud this author on 1: making it free rather than freeium-SaaS - locally I can run my own instance and if the developer decides to abandon it via an Apple Acquire-Hire-Shutdown (FoundationDB where are you now?) my data and workflow aren't lost, 2: exploring a new paradigm (PIM keeps on being re-invented because no one has sufficiently solved the problem-- this was one of the few actually innovative takes on PIM), and 3: specifically because of how open it is and even if I want to use this on a VAXstation from 1993 I can easily compile it myself.

TreeSheets and Leo[1] are the only two PIM's other than Emacs Orgmode[2] that have genuinely been flexible enough to capture information in a flexible enough way for me to 'brainstorm', while concurrently allowing ease of recall (though the new KDE Suite of QT apps are decent enough)

[1] http://leoeditor.com/ [2] And even org-mode tends to be a bit lacking out of the box, though the healthy elisp ecosystem around it + your own ability to hack it into whatever you want makes it versatile enough any workflow.