Marking this as important.
Well, I've checked the files. Some files are working (those are made with UE4.7), some - not. Files which aren't working, made with some old version of the engine, and working well if I'll open them with options:
These files were made with 2 different engine versions. The version for files which are "old" is some pre-release 4.0 version. For comparison, release version has package version 342, but I'm setting it to 335 (actually, some number smaller than 336).
In example uassets which I've got, old files has names starting with HF... and RM..., plus bad file is Raider1P.uasset. Other files are good. I have no idea if these files are separated in some way in the game's directory tree - may be they are located in different pak files, I don't know. If they are separated - that's good. If not - I don't know the easy solution for opening anything without having umodel to crash. However I think the files are separated in some way, because Unreal engine can't load packages from 2 different engine versions, so "old" packages are a kind of garbage in the game directory.