When you use a generic term like crab apples, it is difficult to make useful conclusions. There are so many different species and cultivars, and a great deal of genetic diversity. I have bitten into a lot of different crabs, some are so bitter your mouth makes the "cats anus" expression and all the saliva disappears from your mouth courtesy of the tannin. Others are very sweet with just a modest amount of bitterness. Mostly they are fairly high acid, what you could call bittersharp. I pressed some crabs yesterday from a tree that is a seedling of "John Downie" and the sg was 1.073 with pH around 3.5, very nice juice with plenty of body. (southern hemisphere summer here). Crabs can be well worth experimenting with, but there are no hard and fast rules.