Aselsan AESA radar, EW pod, targeting pod, avionics suite...
The Iraqis found out how their US ally slipped them printers that gave away vital information during war. TAF found out from a crashed plane "bugs" planted by their "ally". What makes anyone believe that there are no bugs hiding in the million or so code on an F-16? Bugs today are so tiny they are impossible to comb out from something as complex and large as an aircraft. We can agree to disagree but I do believe that F-16s will be compromised in some way or another.
Even if they are not. Even then, its not wise for Turkey to put all its eggs on the NATO basket anymore, given that NATO is today the biggest financier and trainer of terrorism against Turkey.