"Regrets"? No. Had it come with the truck I probably would have liked it and may even be spoiled by it, but I haven't regretted the 8.4 at all, it's large enough for me and works great. One thing that concerns me about touch screens, is that "playing" with a screen in a moving/bouncing truck is not ideal, I always prefer large, easy to use buttons with real feedback. So if the 12 sacrifices physical buttons, then I would definitely have to think twice before getting it.
On the flipside, I wouldn't worry about reliability of the 12 either, because there shouldn't be any difference there between 8 vs 12 just because the one is larger than the other. Computer monitors or TV's don't become less reliable just because they get larger.