We had our fences replaced (probably about 25 panels, with concreted posts etc.) about three years back. Our handychap did the labour (and he painted them last summer, he did say best to leave them a few years before sealing). I think they were pressure-treated panels, so I assume they impregnate them with some kind of wood preservative. They came from the local hardware place as opposed to one of the big DIY palaces, and were apparently 'made in Britain' (something so uncommon these days, I noted it).
Anyway, I can't say I've noticed any significant gaps or shrinkage in the actual wood, the only problem we had was with a garden gate that warped away from the frame each time it got wet and needed a bit of wood screwed to it to keep it straight.
I'd benchmark them against some local examples of similar fences, but as said, after three years, you're unlikely to get much traction with a complaint.
(Ironically, we did have to replace several interior doors replaced due to warping – buy spendier doors and don't assume that they'll all the look the same under some paint, sigh.)