Just been looking at this. For me, the two drawbacks would be that (1) it seems to work by either email or people logging onto their website, and (2) the website seems to import your Google calendar and contacts.
I have no great problem with (1) myself but a number of musos I play with are allergic to actually, y'know, doing stuff. If it isn't presented to them on a WhatsApp plate, then they won't lift a finger.
As for (2), I'm really not big on conspiracy theory and the whole "they're spying on me" schtick, but that does seem pretty intrusive and very easily sold on to the spam merchants.
Have you any direct experience yet @Rich?