I had the same question. That said, Zotero's sync story is pitiful and that looks like probably never changing; something like this might easily improve on that owing to the stack it's built on.
edit: I may very well give it a try on that basis. I've got a large collection in Zotero on one of my machines, and a partially overlapping large collection in the Zotero app on my phone, and they are partially overlapping precisely because Zotero sucks at sync. It'd scratch a years-long itch to have my collection unified and easily available for both use and new accession, and Jendeley looks like it may well make that comfortably achievable.
edit: I may very well give it a try on that basis. I've got a large collection in Zotero on one of my machines, and a partially overlapping large collection in the Zotero app on my phone, and they are partially overlapping precisely because Zotero sucks at sync. It'd scratch a years-long itch to have my collection unified and easily available for both use and new accession, and Jendeley looks like it may well make that comfortably achievable.