I worked at a Honda-Yamaha dealership from 1977-1983. The only bike that we would not take in for service were TX750's. Period.
I remember the owner and service manager telling me stories about how the TX750's would blow-up motors left and right. You could not get parts because they were always being modified to fix whatever the latest problem happened to be.
Finally, Yamaha came around with a truck and dropped off new engines, free of charge, for each TX750 that the dealership had sold. They replaced the engines and advised the owners to sell the bikes, as we would no longer work on them. Ouch.
I only ever saw one in the 7 years I was there.
'12 XT1200Z SuperTénéré, '09 KTM 530 EXC, '08 Triumph Tiger 1050, '87 XR600R Street Legal
Gone but not forgotten: '92 Yamaha TDM850, '82 XJ550 Seca, '80 XS1100,'85 GL1200A, '84 V65 Sabre, '80 CB750F SuperSport, '77 RD400, '72 TS250, '69 CB450K, '68 CL70