It would seem to me that the biggest concern is the limitations of modular construction--to get the lower price, I believe you are stuck with standard options for things like window size and ceiling heights. If you start customizing the modules, I think the cost would rapidly shoot up to where there is little difference between this system and normal construction.

My neighborhood has a lot of old housing stock, being in a university district, and many brick houses have wooden additions off the backs, so that isn't so odd...what really destroys the looks of the union is when you don't match any details with the main house, such as trim, window sizes or placement and roof slope. There are some truly hideous houses here which were once very nice two and a half storey brick, but to pack in students, have three story decks tacked onto the fronts...be very careful in the details you choose so that future owners will pass on your house because of an insensitive addition. When I was house hunting, I wouldn't even look at a house where things such as window openings were made smaller to fit newer windows, among other things.

You really need an extra 20 feet added to an existing kitchen? What are you feeding, an army? :)