Absolutely some of the most difficult sounds to get right. But not because they are strings or voices, but because they are massed. An average major orchestra can have more than sixty string players depending on the work being performed. One of the things that gives a great string section it's beautiful sound is the fact that you have that many different players, each with his/her own individual sound, playing together to create a "blend" where no one player sticks out (unless the music calls for it) and, as musicians like to describe it, "getting inside each other's sound". The result, in audio-speak, is an incredible amount of low-level information that the recording process, and the playback process, can only hope to get close to capturing fully. Sixty flutes, or banjos playing together would probably tax one's stereo (and recording mics) just as much. Luckily, not too many works written for sixty banjos.