The 'funeral march' impression is interesting, isn't it? Maybe it stems from a "classical music==boring and depressing" mindset, which tends to be more popular with younger people or people who aren't familiar with classical? I can hear a lot of European orchestral and operatic influence in YK songs, and although I still haven't 'got into' opera, there are a lot of orchestral pieces that I really enjoy. There's a lot of stuffiness and snobbery amongst some classical fans sometimes, so it has this serious/formal image that puts people off and isn't really fair on the music at all.
There are a lot of classical fans and classical-influenced composers (YK included!) who know that you can make emotional and beautiful music with classical instrumentation and operatic vocals. What strikes me as weird as well is that there's a lot of electro, dance and rock influence in YK's songs as well! Rock and dance music tends to be more 'bouncy' and 'upbeat' so I guess the people who don't appreciate her songs haven't heard the 'right' songs for themselves yet.
That said, YK can 'do' dark very well, so I think a KnK or Cossette-style OST would suit Heaven's Feel very well. The emotional range of her music is unusually broad though - not all OST composers can go from light to dark while still sounding pretty or beautiful. Type Moon stories have this weird atmosphere where it's brutal one moment and joking the next, and combines contemporary Japan with 'gothic' aesthetics. Hmm...it's sort of hard for me to explain, but a YK music score suits a Nasu story (or an Urobuchi one for that matter) better than one that's a SF, mecha, slice of life or less 'fantastical' one.