I've been using this mod since its creation, long before joining the site, and it has never caused me issues with Facebook accounts, mine or those of people I know. Never, not even once. The mod has an anti-tracking feature, and if you follow the modder's instructions and know how to use it, you won't risk anything. Those who play this game obviously do so for the long haul. Hogwarts Mystery isn't one of those idle games where you use and then discard the game at pleasure, with no one caring about the progress made. HM Is for those who want their Hogwarts adventure, and, obviously, saved progress is a crucial component. Therefore, solving the Facebook login issue is extremely vital because no one downloads this game to throw months of effort ( and cash, maybe ) in the trash for just playing it a bit and then and then puf, bye bye.
The mod Is safe and so Is the Facebook login, just learn how to use It.