It all depends on the nature of the cracks. If a crack follows a panel joint it is probably due to poor installation or use of insufficiently dry framing. Look for screw pops to support this assumption. Cracks in a ceiling are usually due to these factors or unusually high humidity and if the cracks are at the ceiling-to-wall joint it can be because the installer screwed the ceiling edges to the framing instead of allowing the wall board to support them. If a crack is vertical at a door/window corner then the installer probably put a joint there which, of course, he shouldn't have. If a crack is diagonal across a panel it is most likely due to foundation settlement. Settlement should produce cracks in the foundation as well so it is usually not difficult to see. It is possible that the model unit was built to meet a tight deadline and the drywall joints were taped with a fast drying "setting type" compound and painted the same day. If you are really worried you should photograph the cracks and post them here or get a structural engineer or architect to visit the site and report back to you.

In some parts of the country, houses settle more at certain times of year---in New England where I grew up, for instance, it was the spring/summer when it got hot and humid after cold, dry winters. In California where I live now, it's the winter when the air gets moist. So it might just be normal creaks. However---you can have a foundation/drainage firm (or two, just to cross-check!) come by to take a look. We did this before starting a kitchen remodel to ensure we wouldn't put in pretty new fixtures and walls only to shake them up with any foundation work. If it's not for a buyer/seller inspection, none of the firms where we are charge for the look and estimate. If it's a drainage issue from your neighbor's yard, let them know that they need to fix it. If it isn't a foundation issue and is just normal creaks, you can also fix many of these from below by bracing the problem spot. We have a book on rehabbing old houses with pretty detailed instructions for fixing any dips in flooring or creaky spots...haven't tried it yet, though. Good luck!

A properly repaired crack will not be any threat to crack again in a horizontal use situation as long as it's supported properly and that support doesn't shift over time---and it's not an inherently weak stone that is prone to cracking as some of the pricier already mesh reinforced and resined ones are. It will still be visible as a vein in the granite but should be mostly smooth to the touch, similar to a fissure. I wouldn't personally have any fears about installing such a stone in my own kitchen as long as the fabricator was well experienced with epoxy repairs. If you are uncertain, let them repair the slab now before fabrication and see what you think about how it looks. And thoroughly inspect the stone for mesh and resin reinforcing as your choice may be one of the ones that is fragile.

The contractor is what you should be concerned about, not the tile. Smells like laziness - people like this will tell you anything to get out of doing a job that requires anything beyond the basic bare minimum. IMO, they’re showing you firsthand how lazy they are by dismissing something right off the bat rather than educating themselves on the product and its differential properties. There’s nothing worse than someone unwilling to learn. The inability to admit playing a role in a product’s failure is a sure sign of people unwilling to take responsibility for anything, which is definitely not something you want in a contractor!