They don't have to be horrible. Boosey & Hawkes made a particularly nice composite clarinet (many moons ago now - I bought mine more than forty years ago) that

They don't have to be horrible. Boosey & Hawkes made a particularly nice composite clarinet (many moons ago now - I bought mine more than forty years ago) that didn't sell particularly well, since it had the minor disadvantage of being somewhat more expensive than a low-end wooden instrument. (It was also heavy enough that your right thumb would notice after a long session.) Longitudinal glass fibre and some heavy filler in the resin; it was probably a lot more expensive to fabricate then than it would be now, especially if the instrument has a conical rather than cylindrical bore, and I'm sure the materials could be optimized a lot more easily on a computer rather than by the trial-and-error of the day.