Having worked on F-5E's at RAF Alconbury, both flighline and Phase in the 70's, I believe there were modifications made to the Tigers in the early 80's in the tail and nose.  I's possible the Swiss versions didn't have these modifications and must be added to meet the US standards.

In Finnish operations MiG-21s were proven to be as reliable as Western single engined aircraft (if not more reliable, our records were better than in many countries where they have/had Western aircraft), but there is one "but" - a big one. MiG-21 is very maintenance intensive, it requires a lot of man-hours to keep it in reliable condition. If there´re reliability issues, this is probably the most likely one. The mechanics have to be paid, and when we deal with aircraft we ain´t talking about mechanics with low wages.