Thanks for the comments Fred and Jan. IBG's kits are certainly not quick build kits, but I haven't had any real problems with those I have finished before. In this particular case I don't think it is so much over-engineered as I think there is just something wrong with the track - sprocket - road wheel interfaces in terms of design and / or execution.
After writing this I noticed that Russ Amott did a built review of another IBG Type 89, and he reported problems fitting the track to the sprocket to the extent that he cut the sprocket teeth off on one side, and it is visible in the photos on that review that the track links are a bit "mashed and crooked" (his words). It's also noticeable that the road wheels don't quite sit evenly and fully on the track. The gallery on IBG's website has a finished example, and, again, the road wheels aren't quite on the track at some points, and the tracks look decidedly wobbly around the sprocket and idler, where they stand away from the wheel rim, so there's a gap visible (as on the 3D CAD drawing in the instructions). Note too that the top run of the track doesn't sit on the return rollers, but kind of arches over them.
Maybe I'm getting too fussy, but you're not going to win any competitions with that...
Anyway, don't let this put anyone off IBG kits generally, I'm building another one at the moment, and it is going OK, if also not the simplest.