Martin (1992: 202):
Within the framework of systemic functional approaches to discourse, comparative relations are the most controversial category to be developed here; … they are not treated as a major category of logical relation by either Halliday and Hasan  or Halliday . To explore this question, it will be useful to return to text [4:2].
[4:2] p. With the bigger breeds of dog, they're stood on the ground,
q. because it's easier for the judge to handle them.
r. With the smaller breeds of dog, such as Corgis, all the Toy-breeds, Dachshunds and this type of thing we — as our turn comes,
s. we stand our dog on the table.
As noted in Section 4.1 above, there is a clear relationship of contrast between p–q and r–s. This is coded through the comparative reference (bigger vs smaller) and the lexical cohesion (most clearly ground vs table); and the contrast is highlighted by the marked Themes in p and r. As noted in 4.2.4 above the contrast could have been made explicit in [4:2] with a conjunction such as but.
 This confuses two distinct logical relations, whose meanings are N is like M and X and conversely Y. In SFL theory, the former is classified as enhancement: manner: comparison, and the latter as extension: additive: adversative ('but').
 This fundamental confusion is one factor that explains why this approach is "controversial", not least because it undermines the logical validity of what follows.