Vacuumorphs. Not their freaky anatomy, that's excusable. They needed to be functional, not pretty. The problem, though, is that, as a species, they were doomed to failure from the start. Why create them, anyway? They were restricted to a very narrow environment(Earth's orbit, they can't travel with a ship, since the ship's acceleration creates gravity), have no means of self-sustainance(even to breathe they need a ship as an air supply) and, to settle their certain doom, they're sterile. Even if they weren't, most of their organs are synthetically added after birth, so they couldn't create capable offspring. The biggest problem, though, is that it appears Cramlyn, the first Vacuumorph, was created shortly before they sent the genetic engineers away from Earth, so it's not like they could hope to become a self-sufficient species any soon. So why create them in the first place, only to be discontinued, so to speak?
They were created to aid in the construction of the space fleet, which was a one-off anyway, so they wouldn't need them for all that long. Of course, an actual human in a space suit would actually be far more versatile, so it still makes little sense.
The Hitek. Their bodies degenerate because of...what exactly? It is unlikely to be environmental conditions making it inevitable, given that regular homo sapiens sapiens exist alongside them. And though they are capable genetic engineers of creating several new species, they are unable to make their own organs more robust, having to rely on what amount to combined exoskeletons/life support systems.
I always assumed it was a combination of inbreeding and extended lifespans.