I already complained in another topic about the missing sync functionality for external users and still until today I don't understand why this is not available for them. (Update: MS PM team wrote that they are looking into it).
Nevertheless another annoying limitation is (or I missed it), that I can't see to whom I sent the invitation nor a status, a la pending/accepted/etc. as you can see in Box/DropBox.
Beside that when I copy and paste e-mails from Excel, OneDrive sees them only as one e-mail and won't accept, so I have to prepare all the e-mails addresses in Word with a semicolon and space after each and every e-mail address and copy it then.