From what I read on the wiki it had to do with some sort of disease linked to investiture system there, and things went really haywire? Like basically the investiture system there had to do with microscopic organisms. But again that's just from the Coppermind and my memory of that coppermind entry, lol.
I think we need to find out first how humans got to the Rosharian system in the first place. Then, about the events on on Ashyn - was the apocalypse there caused by Odium? I'm not sure they would have artifacts if that were the case - firstly, they'd be refugees. Secondly, they might be afraid of destroying Roshar like they did with Ashyn, and bringing anything associated with Odium. That's certainly an ongoing theme and is mentioned as a big reason that the 10 orders were made for surgebinders, because surge binding apparently(?) played a part in Ashyn's destruction.
More knowledgeable folks may correct stuff in that but it's my understanding so far