My understanding, from the period I was flying them as a student (1985-87), was that it was a qusetion of excessive fatigue on the empenage assembly/rear tail boom. Given the relatively high GH sortie use these a/c had compare to the civillian market, it seems sensible. The g limits were lower too for the same reason.
Ever the by the book student, I tried it a few times and in reality it wasn't worth it. Although the a/c was a good basic aeros machine (I won the 87 Reid trophy in one, so am particularly fond of it), it can't really be considere as a truly aerobatic one. You couldn't really do much in it by pushing forward either: if inverted you couldn't get it to climb (it just mushed and then you ran out of your 8 seconds inverted time). I tried pushing out from a stall turn recovery and had a "I learned about flying from that" experience.