This was a while ago, but IIRC there were some fairly major software-reliability rough edges (frequent crashing/bugs/etc). Beyond that the major shortcoming was that it was constrained to pdf's only, and that's only one part of my reading workflow.

For a while I was trying to sync a Zotero library (since I need citation management when writing papers), and do all my academic reading (which is mostly pdf's) in Polar, but it was just a bit too much overhead to stick with.