ASPC/AMHR horses are where the market is going. In fact, it's already there. Just look at the National Grand and Reserve AMHR horses over the last few years!
With ANY breed, especially, one's that are hot, you will eventually turn out mediocre horses. The ASPC/AMHR horses that I have been seeing are great quality, however, I have come across some for sale that weren't. I believe where we'll see a problem with these horses is when they are crossed onto mediocre miniature horses. However, the individuals that I know that are purposely breeding for double registered shetlands are very strict with their breeding practices and the quality of horses they are crossing and producing.