The OCaml manual has a chapter about this; in short, you’re meeting the value restriction because the 'a in 'a ref is an invariant type variable, so it becomes a weak type variable '_a when trying to “return” a unit -> 'a ref from failwith. This doesn’t happen with the 'a in 'a list because it’s covariant, which have relaxed restrictions.
This restriction exists to prevent you from using a global instance of 'a ref as both int ref and string ref at the same time, for example. Your function will work if you use failwith inside it, however:
let create () : 'a t = failwith "..."