It couples things and complicates things. Which creates friction against when refactoring a software architecture, making it harder to scale it up.
Fully agree, for locks or other simple begin/end pairs it doesn't create the same kind of architectural damage. But I don't use it even there, because RAII fundamentally doesn't play well with manual cleanup code. It's kind of an all-in game, if you use some RAII there are strong forces (whenever you need to get the "nesting" right) to convert all remaining manual begin/end type code to types that define that functionality out-of-line in the destructor/destructor, leading to fragmentation.