Not knowing the tune is fine for jazz. The best jazz musicians know their tunes thoroughly, that is from a melodic and harmonic standpoint with the right bass line as well.

The trombone is a beautiful instrument and in the hands of a skilled player can make an ensemble regardless of what it is come to life.

You might as well criticize squeeze boxes as wheeze boxes and fiddles as chicken scratching.
As the old cliche goes, "It's the player, not the instrument".

There are times when a rough sounding trombone sounds good as in a trad jazz band.
There are times when a wheezing accordian can sound plaintive.

Playing was is appropriate is more important than what instrument is being played.