Not to minimize the hard work and effort that goes into any CGI production, but it's a bit easier to convincingly animate primarily black & white spaceships (mostly composed of basic geometric shapes), especially when there is no 'real world' basis for comparision for the viewer
The challenge for a film like Red Tails, is that almost everyone has seen actual WWII film footage and have a somewhat prejudiced view of what P-51's and B-17's should look like. That plus need to create organic backgrounds (trees & grass, etc) instead of the stark background of space, as in Star Wars, etc.
All that being said, as the technology continues to improve I'm sure the 'reality gap' between CGI and the real world will continue to shrink.
Mark