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 impossible to make it perfect, but it's trivial to make it better. For example, get rid of Silvery Barbs.
And we'd really hope with a large corporation behind it, they could do more than get rid of the obvious. They could do the playtesting necessary to properly balance martials vs casters.
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.