Flute playback vibrato is too much
The flute's vibrato is obstructive to its sound. It's very distracting. The default sounds should be straight tones or subtle vibrato, because vibrato really should be only an embellishment rather than dominating the sound. None of the other wind instruments are as bad as the flute.
It would be nice to have a subtle and controlled flute sound.
The flute playback in Noteflight is a good reference, in my opinion: that one uses subtle vibrato that doesn't get in the way of the music.
Would it be possible to add a feature to add/control the vibrato used on each note, like a performer does?
If all the musicians recording sounds for MuseScore recorded straight tones, then the default would be straight tones, and there could be a feature for composers to select a note and add vibrato: they could control the frequency and depth of the vibrato, and I imagine that would just be a function of pitch adjustment.
If that's too much or too complicated, then I think the standard playback sounds just need to be more subtle.