because most banjos are out of tune

God knows there can be little excuse in these days of electronic tuners, though I do admit there will always be a wayward harmonic resonance that might sound odd to certain ears, but that's different from being entirely OOT (expressed in my native Geordie tongue as well as a serendipitous acronym). The stability of modern banjo heads is beyond doubt (we use Remo plastic fiberskyns on both banjo & frame drums - note: frame drums not bodhrans) - though I've no doubt you speak from long years of bitter experience. What irks me most in this respect is that WE FOLKIES are supposed to cherish such earthy homegrown irregular filthy resonances which are the heart and soul of the Banjo (and bagpipes / hurdy-gurdy / melodeon &c.)

The sound is usually horrid - a violent attack with no tail or sustain

IYHO, Richard, which is fair enough, but the delight of a banjo lies in its multiplicity of timbres each of which is unique to the instrument and its many & various playing styles - and the players thereof - from piano to forte and all points in between. The sound which so offends you just happens to be one of my chief delights.