Apparently there is a thing called thatch, which is bad for lawns, which sod is much more prone to: I get the impression that it's tangled-matted stuff that blocks water from getting to the roots, and sod is kind of insta-thatch by its nature, so getting the grass to root into the soil below the sod is tricky. My lawn guy said he'd worked on lots of lawns where they did the front in sod and the back in seed and the back was much healthier.

OTOH, he did say that if we hit the lottery and wanted to pay someone to dig up the entire yard and put down sod, well, hey, it's your money.

However, when I heard that we couldn't just lay sod down on the little paths the dog has worn in the yard, but would have to dig up those areas and fit the sod in, my interest dropped to zero immediately, as you can tell by the less-than-clear explanation here.