The data is not likely to be truly anonymous in practice. Making sure it's truly anonymous takes more work and makes it less useful than not-quite-anonymous, so

The data is not likely to be truly anonymous in practice. Making sure it's truly anonymous takes more work and makes it less useful than not-quite-anonymous, so there's no incentive to do a good job of it. Instead you end up with e.g. Microsoft's constant stream of "oops, we forgot to inform you of the data collection", "oops, we collected more data than intended", "oops, we sent it to our servers accidentally", "oops, it wasn't really anonymous" (e.g. because they simply hashed an ID to anonymize it), and so on.