URLs uniquely identify bookmarks, if two chromes have a bookmark with the same URL but different titles/tags, stashmarks will show whichever was synced first.
If I edit a URL, stashmarks doesn't reflect the change in the UI, at least when the original is still pending download. Looks like it's actually changed internally.
Also, I can't delete (or edit) a bookmark in stashmarks that I no longer have saved in my browser. Recreating it and deleting that does nothing, even when logged out and then in.
Hope these critiques are helpful.
Clearly, these are all edge cases, and overall your service is impressive.