Martin (1992: 260):
The critical point is that because it so closely modelled on clause complex interdependency, Rhetorical Structure Theory cannot at present handle simultaneous conjunctive structures in text. Since internal and external conjunction very commonly assign simultaneous layers of structure to a text, much as experiential and interpersonal metafunctions in the grammar do for the clause, this short-coming is a very serious one indeed.
 As demonstrated in previous posts, the "simultaneous" conjunctive structures only arise from theoretical misunderstandings of Martin, such as
- mistaking parataxis for addition,
- misapplying the logical semantic category concessive,
- mistaking interpersonal functions for conjunctive functions.
 As demonstrated in previous posts, Martin has misunderstood Halliday & Hasan's (1976) distinction between internal and external conjunction throughout, misconstruing it as the distinction between 'constructing text' and 'constructing field'.
 An important difference here is that the experiential and interpersonal structures of the clause are different metafunctional angles on the same unit, whereas any simultaneous conjunctive structures would be from just one angle: Martin's logical.
 Given the above, no shortcoming in RST has been identified, serious or otherwise.