(Sorry for the delays in the next part of Darkness Within, folks – having some trouble getting it to gel in my head.)
The Unpronounceable: Quite a few, due to all those species that don’t use spoken language the way we understand it: as mentioned previously, esseli names are DNS strands, myneni names are made of sounds only a synthesizer could love, galari names are modulated EM radiation, and then there are the sonar pings, electrical waveforms, patterns of bioluminescence, complex aromatic chemicals, neural-gestalt-expressed qualia which are very, very hard to parse for anyone of different brain design, etc. All of that is before you get to the really simple problems like different larynx designs.
It is somewhat averted due to the Eldraeic language being designed as a lingua franca, and thus possessing multiple different phonologies for its alphabet, including several designed for different ways of speaking, including sonar, bioluminescence, EM codes and DNA encoding, so in theory it should be possible to transliterate names encoded in those ways into something pronounceable and adequately unique, even if it doesn’t resemble the original all that much to the ear or other organ.
In practice, not so much, or not without your speech organ hurting, or not in a manner that’s agreeable to the person named. But you can try, at least.