They have a very suspect backline. Licka would have been so dangerous, but I'll back Gumby, Hurley and whichever other talls are fit to monster a reasonably poor Thompson/Grima combo in the air.
From what I have seen of North of late, I would say Thompson and Grima are very underrated. They get the job done without fuss and without taking your notice that much. I would happily have them in our team. Marking forwards regularly play well against us. Our guys concede more uncontested marks to key forwards than most good defences. By uncontested I mean marks like Jenkin (2) and Walker took on Sunday.
(NB am I allowed to call him Walker without putting Taylor in front, ala "triple Brownlow Medallist Bobby Skilton"?)