"The only real downside is the mess of external lubrication and the need to re-lube at short intervals."
Also having to replace sprockets and chains at intervals is a hassle. I have 130,000 miles on my shaft Honda Sabre. I probably would have replaced 6 set of chains and sprockets by now. Also lubing and chain tightening is more labor intensive than the every 12K easy oil change with the final drive.
2000 Honda Sabre VT1100 (194,000 miles)
2005 Suzuki DL650 (116,000 miles)
1976 BMW R90/6 (33,000 miles)
2009 Vespa GTS250ie (8200 miles)
1993 Honda CB750 (13,000 miles)
2013 BMW R1200GS LC (16,600 miles)
2015 Kawasaki KLR650E (3,200 miles)