It really depends. Sometimes I start with one instrumentation and then change to another. Like my Weather Music Suite for instance, it started as a piano solo, then got up to a chamber ensemble of Piano + String Quartet + Flute, and then finally full orchestra. Or my Symphony no. 1, which was first conceived at the piano and became a full orchestra piece. I even have a PDF and MP3 I made of my second theme ideas for that symphony(which is mostly piano with commentary), eventually settled on the first one. However, often, I think of the instrumentation first, like I'll want to write say a woodwind quartet piece and then I'll write the musical material for said instrumentation.
Kind of both? I'm obviously concerned with the musical material. But I also like experimenting with different instrument combinations, like I did for Flute and Cello or am doing for Flute Trio(as in a trio of flutes, not a trio with a single flute).
Yes and no. Yes in that I'm satisfied with writing some or even a lot of repertoire for standard groups, but no in the sense that I wouldn't be satisfied if that's all I wrote.
Yes, I've felt this several times, like with the Weather Music Suite I already mentioned.
Yes.
I don't know, I've never really done such a thing.
Sometimes, especially if I'm trying to write a solo piece. Like Cello for instance. I have no problems really writing idiomatically for Cello as part of an ensemble, but solo Cello, I do have difficulties with.
Underrated, I think the Woodwind Quartet is an underrated ensemble. Overrated, eh, I can't think of any. I don't think the orchestra and string quartet are overrated personally.
Most challenging, definitely quartets, especially string quartet. Piano Trio also as that's essentially a quartet at minimum in terms of compositional density. Also any non-keyboard Solo, as with that, I have to think about how to get melody, bass, and figuration into a single line or maybe at most 2 lines outside of chords for strings. Least challenging, trios(those without piano anyway) and orchestra. Trio because I feel it's perfectly balanced in terms of instrumental roles, I never have to double roles with a trio. And orchestra because of all the instrumental colors. Just a couple years ago, I probably would have said Trio and Solo Piano, but ever since I've become comfortable writing for orchestra, it's become just as easy, maybe even easier than Solo Piano for me to compose for.
Piano for solo, otherwise, not really, I compose about as much for orchestra as I do for chamber ensembles.