It's implied when Azmuth said he had a safe place to send the Sentient Ultimates he was talking about Primus, since there weren't too many other places he could've sent them. I suppose they might've been sent somewhere else, but it's a pretty fair assumption that Azmuth sent them to Primus.
Since Eunice still exists after So Long and Thanks for All the Smoothies, and she lived on Primus, the Sentient Ultimates might still exist. They've never appeared outside of their debut episode though, so it's difficult to say.
It would be nice if they touched upon their existence again. To be honest, I was hoping they would get names eventually. Being called "Sentient Ultimate Something" just seems like too much of a mouthful.
I agree, the Sentient Ultimates really got the short end of the stick, although I understand why they were never touched on again (it was kind of a one-off episode). Unfortunately due to the Original Continuity being effectively concluded, I don't think there's any chance they'll receive more development.
They could've been sent to Primus or they could've been sent to the devolved forms home world or Azmuth might have found a completely new planet for them to live on. I have to agree that the most likely place is or was Primus, and since Primus is no more after the recreation of the universe then they had to have been either wiped out or moved to another planet.