The NK650 is a very under rated machine and motojournlist Allan Cathcart rates them very highly.
I have a TK650 (same running gear with 'touring' bodywork, pannier, etc) and with 10,000 on it now have had to do nothing other than change the oil and today fit a new rear tyre.
If you like it and suits your needs then buy it, ride it and don't listen to the opinions of those who have never ridden one.