Glass tile is completely straight. If there is a bow in the set it will because the wall is not plumb. Glass tile is not an easy set, he may be trying to discourage you from using it. It is difficult to cut, you have to use a VERY sharp blade or the glass will chip at the edges I have used 3 x 12 and 4 x 12 tile without issue. If you have cut edges at an end run that is exposed, you may need metal finish edging called "Schluter". This is available at tile showroom.

Thanks Lauren. What he did was put 2 pcs back to back and the ends were flush to each other but in the middle you could put a quarter between them. Sort of like lippage in large format floor tiles, he said in a 1/2 offset layout the low spot would butt up to the high spot of the other tile. Am I making sense?

I’m having the same issue with my 3 x 12 glass backsplash. When placed back to back you can see they are bowed in the center not completely flat. What did you end up doing?