Carbs had problems, requiring a rebuild because jets were clogged. This happened because the bike had been idle for almost two years, and the jets became gummed up, so it would start missing on a cylinder.
Disk brakes required a complete rebuild because they got hung up. The dealership said people just replace them at over $235.00 a piece. I did not, but instead separated them into halves and cleaned out all the built up crud from hardened old oil. I used a rebuild kit, which includes the gaskets and seals. Cleaning and flushing, and they work great.
Misfires caused by the plastic casing on the coil developing age cracks would let moisture in. I bit the bullet and replaced it, and put in new spark plug caps. Now runs great. It was cheaper, and fit fine when I used straight plug caps instead of the 45 degree angle caps.
One other trouble I had was instruments failing from time to time. It turns out the ground for them is a wire that is held by the same bolt that holds the thermostat to the frame. The bolt can't fall off, but it can come loose. Put on a lock washer and tightened it up.
Always check the battery water level a minimum of once a month. Always pull the battery out and give it a slap to get the air bubbles out of it. Recently after a hot month of summer driving with a new battery, the slap made it go from the top of the low water mark to less than a 1/3 full. If the battery is full to spec and properly charged, you will not have trouble starting it right away.
Remember my troubles were over 24 years of single person ownership. It had been stored outside in New England weather. Only in the last 4 winters has it been garage stored. I had the seats redone, and since I was in no rush, the guy charged me $125.00 for both seats and the sissy bar.