Martin (1992: 391):
Within discourse semantics, the ways in which systems co-operate in the process of making text is much less well understood. … A more explicit account of this co-operation is clearly an urgent research goal; English Text has been concerned not so much with addressing this goal as with making it addressable by proposing four relatively independent discourse modules to beg the question… . The point is that integrating meanings deriving from different metafunctions is not a task that can be left to lexicogrammar alone.
 This continues the misconstrual of the metafunctions as modular units, rather than as complementary perspectives.
 This misunderstands the meaning of 'to beg the question'. Begging the question is a logical fallacy identified by Aristotle in which the premises include the claim that the conclusion is true or (directly or indirectly) assume that the conclusion is true. See further here or here.
 This misunderstands stratification. The wordings of lexicogrammar realise the meanings of semantics; the meanings of semantics are realised by the wordings of lexicogrammar. Further, it ignores the trinocular perspective on which linguistic analysis is based in SFL. Each stratum is viewed 'from above', 'from roundabout' and 'from below'. This includes looking at lexicogrammar 'from above' (in terms of the meaning being realised) and looking at semantics 'from below' (in terms of the wording that realises it).