There's a large development near me that's been built over the last 5 years and recently finished. A lot of the homes have now gone up for sale again. I don't imagine I would buy a new built but who knows. The downsides I have heard from a short conversation with someone living in this new build estate are:
Company promised lots of little playgrounds etc and have been hassled for years until finally building them. Several of them are very naff and not something you would look at and think "that's a playground" at all... They've also had to be hassled for loads of other things like planting trees etc.
Narrow roads, parking problems. Whenever I see a parking thread on here I think, here we go, a new build estate I bet! The one near us has a few signs up which frustrated home owners probably made after arguments with their neighbours about their shared access driveways.
It can be nice that everyone moves in together and you can get more of a community feel. But it sounds like often a WhatsApp group gets set up and it's full of people moaning about the building company and posting pic sof suspicious looking people on the estate who are probably just normal people going for a walk.
Tiny gardens with high walls = dead grass and eventual mud bath. The land is often quite boggy as if the land was great for building on, I suppose maybe it would have been built in already.
Small rooms which can only be configured in one way. No storage space. Even radiators have no gap between radiator and wall in houses I've seen, creates more space but you can't dry a towel on the radiator which must be pretty frustrating.
Paper thin walls. They don't use breeze blocks.
Tiny windows. Although it's cheap to heat up the house, I've known people in new builds still have the heating on in May because they're cold. The windows are too small to let in much sun light at all.