Currently, all identifier types appear valid to set on all entity types (or at least, I found the same list for both Publications and Works). This could lead to unexpected identifiers being set on entities. For example, an Amazon ASIN values, which presumably identifies a specific Edition, may be attached to a Work which has several Editions by way of one or more Publications that (presumably) contain it. Similarly, a Publication or a Work may currently be given a LibraryThing Author identifier, which has no meaningful interpretation in that context.
BookBrainz should constrain the list of identifiers to only those that are relevant for an entity type so as to prevent such nonsensical identifiers from being assigned.