I can read some suprising comments here
So horizontal scanlines aren't a good raison to complain since :
1 - if you bought such an expensive monitor, then you can afford gpu's to keep a constant 100fps and "see? there's no scanlines anymore"
2 - g-sync is only useful when running at a rocksolid 100+ fps (in that case, I prefer ULMB, just my opinion...). So we have to play at low/medium settings in order to avoid fps drops meaning scanlines.
Personally, I bought X34 precisely because g-sync is far more useful when fps are between 35-55fps and allows to keep a smooth gameplay in demanding games at such a resolution. And not to mention those games which aren't SLI compliant.
These days, I play Witcher 3 everything maxxed (+ some CFG tweaks) with oc'ed 980ti SLI and at Novigrad, I'm really far from getting 90-100 constant fps but 55-75 and yes, I can see scanlines on the entire left side of the screen.
I don't see the point to lower settings in that kind of game to get constant 100fps since it's a perfectly enjoyable gameplay even at 45fps+Gsync, with sumptuous graphics.
Same for GTA5 and probably Fallout 4 soon
That's my opinion but even without considering that, we're talking about a 1300€ monitor and it's not acceptable that such a defect occurs when running the monitor at its official specs.