are there people unhappy about living in the fortress towns, for instance?
Almost certainly. Fortress towns are still warzones under regular attack from nod and/or it's still a yellow zone with ion storms, tiberium, and all the attendant problems therein.
We should at least take the question seriously.
One thing I'm considering is the lore aspect of housing.
Like, currently, One thing people want is cars for the last leg of their journeys.
Now, IRL modern america is built around the car with large sprawling suburbs and rural areas without good public transport and rail links.
GDI on the other hand, has very compact dense cities in the blue zones, where a lot of people are packed into high density housing and arcologies which are all in one housing developments. So a lot of people have things very local, home, work, shops. So don't need personal transport, along with good rail transport and (I think) bus services. And stuff is either in walking distance or can be gotten to using public transport.
Rural areas, effectively yellow zones in our case, particularly fortress towns, well. The general public there, again, have everything packed into a small area and simply have little need or desire to leave the bubble of safety the fortress provides because outside the walls is a case of "here be dragons" said dragons in this case being tiberium and nod flame tanks.
Sure some motor vehicles would be nice for those who need them, but as is with the arcologies and high density duplexes combined with good quality rail, they're not really needed. And without large amounts of traffic the cities are cleaner, with less traffic and without long commutes and morning rush hours, at least, not those involving heaps of vehicle traffic.
The suburb is a nice ideal, a family home, with a nice picket fence and room in the garden for kids to play. But it's very space intensive and costs in logistics and the like quickly accrue.
I'm not inherently opposed to housing grants and the likely result of that. But in my humble opinion such housing should be a supplement to our own efforts, rather than the main source of it. Tiberium being what it is, the bulk of our housing should be well defensible and concentrated. Beyond that, I enjoy the concept of arcologies having everything within easy walking distance, and the resulting lack of need for motor vehicles. Most if not all of our cars would be electric of course, cutting down on pollution and those who live near rail lines likely need good sound proofing. But there's definitely something to be said for less noisy cities.