Judd, I have found that some aircraft simply have pretty average autopilots - they're not suffering from a "fault" as such, they're just not that flash!
The CL604 couldn't maintain the localiser (particularly well) with any more than an 18 knot crosswind - it wasn't broken or faulty, it was how it was...
What's with the whole high and mighty "Naturally, you would have done the right thing by writing up the perceived autopilot defect in great detail in the maintenance document." attitude?
(Interesting note - both aircraft produced by Bombardier!)