The dwarf issue is debated at least once a year here on the Forum. It is a major hotbutton issue with many people and there are lots of opinions, conjecture and some solid facts but very little research so far. There has also been talk of research into the dwarf gene, etc., but I haven't heard if it is moving forward or not.
I'm of the opinion that not all dwarfism is genetic and much can be environmental - nutrtition, toxins, etc., plus some eggs/sperm may not be perfect and result in an abortion along the way - is this a 'dwarf issue'? I don't know. I won't throw a horse out if it had a dwarf, but I would be scrupulous in investigating it's other produce, parents, siblings, etc. to see if it had occurred elsewhere in that line and would not repeat the breeding.
You also need to consider, in older breeding animals, as in humans, the chances for defects are higher. I haven't seen that with our horses, but it would make sense to me at least.
So, I'm not ready to label a horse or line a 'dwarf carrying line' without lots of research.
It just pays to know from whom you're buying a horse if it's for breeding, and to really research available information from the owner, studbooks, past owners, etc. if possible.