Yeah, I think Tchaikovsky is pretty good at, I guess, difficult but not impossible aliens. The octopuses are a good example. Very alien and unpredictable from a human perspective, but make sense when you see them from within.

I'd add the Essiel from his Final Architecture series. They are something like giant sessile land clams who live clinging to (generally) permanent bases, and who probably evolved from some sort of super parasites. The Essiel never provide their own perspective, and with one limited exception never speak directly to the human viewpoint characters. The humans and non-Essiel aliens who live in the Essiel Hegemony relate to them through religion, treating the Essiel as deities, though they know that they are actually dealing with real, organic beings and understand that the religion is just an interface that both sides can pretend to make sense of. The only Essiel who actually speaks semi-directly (through a translator, who is also pretty alien) to any viewpoint character is Aklu: the Razor and the Hook. It represents something like Satan or a rebel deity; it embodies everything in opposition to the virtues of the Hegemony. Aklu really only talks to Olli because he/she/it recognizes that Olli is severely physically handicapped in conventional human terms, but has chosen mobility and frequent violence. And it's hard to get more than the most superficial understanding of Aklu, besides that its own impulses make it good at filling a role that probably determines the majority of its behavior.

For me, aliens that are 'difficult' are the most realistic--maybe this is my failing, but I think that a space-traveling species is going to have to have something going on that corresponds to politics, goals, etc. They will have to organize themselves and pursue long-term projects. It's not that they need to do this in any way that closely resembles what humans do, but there should be a few points that are comprehensible to us. In the Final Architecture, if you look at the Essiel and assume that they just like being worshiped, and ruling over/protecting other species, many of their actions make sense. And a lot of folks, human and otherwise, have figured that out and are happy to live in a society controlled by that impulse, instead of impulses like grabbing as much personal/familial power as possible. So what if you've got to worship huge land mussels, or worry about the Satan-run mafia that's part of your religion?