Given proper fuel supply and cooling you should not have had much problems with the NT. It's rare to have a dud. I have several NT's,they are powerful but a tad heavy. The stuff I had some minor issues with were mainly evident at higher than normal rpm use. The long muffler screw loosen or break, the throttle arm would loosen where it's crimped on or staked in place, the tip of the throttle stop screw would break off and render the carb useless and in later models when they went to steel cage bearings the life was reduced.

Could you have had fuel foaming issues? Was the engine in a cowling with plenty of opening for the hot air to exit? If the next engine has problems, you'll know it's most likely your setup in the plane.

I couldn't find much differences NX vs NT....the color of the head?