OP here. I think another part of the problem besides the browser implementation problems pointed out in this study are privacy protection scripts which block anything that looks like tracking. Navigator.sendBeacon() is often used for tracking so it's an easy target.

This has serious implications for search engines using parallel tracking, which is all the big guys at this point. Parallel tracking relies upon the sendbeacon method to send tracking data. Half the clicks made by parallel tracking calls aren't getting through in our experience.