This has been a soft rule-of-thumb for a long time, because struct/immutable will basically just work like a C struct (that was even one of the minor arguments for the immutable → struct rename), but mutable struct requires more thought. There are various issues like: a mutable struct can’t be stored inline even if the fields are otherwise pointer free, unsafe_store! isn’t available, and they can’t be passed by-val.
That said, I don’t think it’s the problem here unless with with_kw macro is doing something really unexpected.