Yeah they ARE pretty tough. I got a 79 that was "abandoned" in a barn in about 81 sat untouched til 2014 It had no title so I pulled the motor, stuck it in a different bike, did basic maintenance, oil, filters, valve and cam set, head stud retorque, carb overhaul. I have put 5K miles on it in the last two years. It's not babied either. Runs like a Swiss watch, well it is an XS650 it's not THAT smooth. Most barn bikes come back to life with little more than a thorough clean up and maintenance. I've fired up and run at least 20 XS650 barn bikes, most have had a ten year or more lay up. Few had had decent maintenance in their past lives.
Most common "needs engine work" problems are hardened, cracked valve seals or rust in the top end, that's usually bikes that sat out exposed to weather.