I would argue the Doomforge are probably the only halfbreed success story.
However, on another note the racial blessings/curses are in fact all rather ambiguous. For elves that are meant to be infertile and limited in the number of children they have they seem to find it bloody easy to have children. They live potentially ever yet only ever 15 years they can have children. This provides them with the capacity to provide over 66 children in a lifespan and clearly shows that they can become one of the most prosperous races.
Either Iblees was incredibly inefficient at cursing elven bollocks or it might be a little PG and deserves a harsher cursing in the future.
As a side, humans being able to serf the seven skies isn't their blessing at all, it is fertility, every other race has inate issues with the production of children. Dwarves and Uruks have no women, elves were cursed with infertility.
But on a note that I would like to get answered in this thread, what is the dwarven blessing meant to be? A strong mind and body is able to be twisted in a hundred different ways. I had gone with the fact that dwarves have a inate ability to show great inteligence, and have always treated my creature like it works in a manner of clockwork, unbending but capable of brief brilliance.
So basically what I am saying is that the dwedmar are a race of savants with mild social issues akin to Austism. (I am not saying that as some harsh joke, I mean it.) They can be brilliant, performed some of the most amazing feats in the ancient times such as a successful war against a Aengul and construction of the seal yet have strange and weird values and tend to be unbending and unwilling to accept change.
A clarification on dwarven blessings would be lovely.