my answers in this thread will be mostly based on Daughter of the Empire and Servant of the Empire, which I reread recently.

In Servant of the Empire, there's a Midkemian character (Kevin - I don't believe he appears in the main series? but I could be wrong) who serves as a nice contrast with the Tsuranuanni culture in which he appears, and helps the protagonist Mara to question some of her culturally ingrained views and prejudices. So far, so good - it's an interesting description of a culture clash, though Feist and Wurts don't go into detail as much as they might have, and Mara's "conversion" goes perhaps a bit too fast - but in terms of worldbuilding, those scenes don't reflect too well on the Midkemia worldbuilding. The problem is that Kevin comes across like a modern American at the core, covered by a thin layer of western medieval notions (his name, incidentally, most certainly does not help with that). I imagine in Riftwar itself this is perhaps less painfully obvious because they don't really have the contrast with the Japanese/Korean Tsuranuanni as much, but still.

I don't remember enough about the other races on Midkemia to comment on them, though as AO says, the alienness of the Valheru was well done. I liked the concept of that gallery with all the worlds, too (is that even in Magician? or only in later books?), it's kind of Erikson avant la lettre.