I haven't bought a Stephen home but I have heard of them. I remember they built a large development in Cupar some years ago and the houses seem very popular. As they are Perth based, and not one of the national housebuilders (like Persimmon and Miller), they're probably not so well known in the central belt.
We have bought newbuilds twice from Morrison and Persimmon, most recently 15 years ago. There will always be snagging issues, and they recognise this by giving you a snagging form when you move in, but in my experience builders were quite good about fixing things, usually quite quickly as in both cases they were still on site and it was more little bits and pieces that were missing.
I remember in our Persimmon home one of the toilet seats was missing, (they just went and took one out from another house!), they covered up small settlement cracks above door frames, adjusted doors that weren't closing properly, installed the outdoor tap which we'd paid for but they'd forgotten, smoothed out bumps in the vinyl and came back after some months and sealed around windows where there was a draught. Of course, you have your NHBC 10 year guarantee too should anything major happen. You also have the advantage of all new appliances, boiler, hopefully better insulation etc.
I would definitely buy a newbuild again. There is something nice about having a brand new house that is yours and not feeling that you have moved into someone else's home, decorated to their taste. After leaving our newbuild it took quite a long time to feel that our next house was actually ours.
With a newbuild there is some extra initial work and expense with buying floor coverings, plants for the garden, curtain rails, blinds, lightshades, coat hooks, toilet roll holders, and all the little things that you would probably inherit if you don't buy a newbuild but you would possibly want to change half these things anyway so arguably better to start with a blank canvas. Be clear though about what is included in the purchase price as Persimmon used to have a system called 'Finishing Touches' and we had to pay about an extra £10,000 for floor coverings, turf, appliances, gas fire, proper fencing etc.
Of course, some people may be unlucky and have major problems but equally if you buy an established house, you could have problems too with big items like windows, doors and boilers needing replaced in the short to medium term.
There will always be a certain amount of upheaval and stress involved in moving house but if you see a newbuild in a location that you like, I would say go for it.