The big thing is ingame UI since the maps are already made. The problem is not the high resolution, the problem is the status part would be way longer and make the whole UI looks strange expecially there would be a blank area around the unit image. Tho I don't know which are more Ugly.
For the android phone, they are mostly 16:9, so it would be too much work once it's set up(still can see the tiny black edge due to there different type of 16:9 screen). But iphone has a extreme different resolution. 640 × 1,136 is a disaster to UI guys. It would look way longer than other 16:9 since in Android there are some icons at the bottom(80 pixels in 720P) which makes it actually a 16:10. Normally in fanta Status takes 1280 - 720 - 80 = 480, 480/1280 = .375 of the screen(and I assume the map is a square). But in iphone 5, the left over space should be use as status area is 1136 - 640 = 496, 496/1136 = 0.43. So in IPHONE 5, there would be a lot more space at the bottom I guess this is one of the concern Fanta developers have(Although I think lazyness is still the bigest reason)