I really hate whenever I try to explain how some bad rules can be abused and immediatelly get someone say shit like "If this happens in your group, change it" as if that would solve the problem. And whenever it is not soemthing you witnessed personally, then it means it never happens and could never happen.
It's a sourcebook they charged money for. They couldn't have bothered to do basic playtesting to earn that money?
And there's plenty more where that came from. Between a Shadow and a Tarrasque, one can safely be beaten by a low-level party, and the other is a threat to the whole world. The CRs reflect that, except they're backwards.
In fairness, caught early, Shadows wouldn't need a level 20 party to stop them. But they're still above CR. And with the Tarrasque, all they had to do was leave in the anti-cheese measures they already had. And steal all the immunities from Pathfinder.
Speaking of CR, that was a bad way of doing things. Sure it's convenient if you have a party of four players fighting a monster, but if you have to figure out how to recalculate it based on different party sizes, you may as well just use level to begin with and then figure out what level would challenge your players. Then it would work just as well on enemies with class levels. And it would mean Polymorph could be at least somewhat close to being balanced. As it is, a single spell can turn one party member into a monster capable of challenging for characters of that level, and then when defeated, they still just turn back.
Oh yeah, not to defend it because it is OP as hell, but there's a lot more at the core of 5E that needs fixing, it just seems odd to single out a spell from a sourcebook most tables don't use anyways, given that it's setting-specific and was never compiled into later core books like Tasha's.
To me it just read more like MTG power creep making its way into an MTG setting. I don't know if that indicates that it was developed by a separate team entirely but the entire thing was definitely just a cash grab to leverage their other properties.