I liked the end to D3 because it wasn't a sequel hook, even though they were planning a sequel. Considering that the sequel was never made, I think it was a good place to leave off the series. Though there were a few things left unresolved, like the origins of the 'alien' virus -- I'm exploring those. The virus as a sentient preadator that preys on entire civilizations, an entity whose sole property is sentience -- a hive mind of sorts. The idea being that the 'aliens' and the 'virus' are not two separate things, the alien is the virus.
Wasn't that merc level on the alien planetoid supposed to deal with that?
They also reveal that the 'alien' planetoid wasn't alien at all, it was a human project that went horribly wrong because of the virus.
It did say in the briefings that it was the source of the virus.
There is a lot of implied story in Descent, especially D2 -- but it's hard to figure out what it means.