I don't have anything quite so old as many folks here, but I do have several TMCC Hudsons and Berks from around 2002-3 vintage. All of these but one were bought used in the last few years, and none of them have had any problems besides having to replace traction tires. The one I have had since new was bought as 'command ready', and I've run it hard, pulling heavy loads for more than ten years now on tight, o27 curves as a conventional engine. last year I finally opened her up and popped in an R2LC board, all the rest of the electronics stay the same, and it has run flawlessly since then, still pulling long, heavy trains of post war cars. As a note, on occasion, for no particular reason that I have been able to determine, these locomotives will stop playing sound. When this happens I have to remove power and restart the locomotive, and everything works fine again.
The only electronic components I've had fail in my o-gauge trains are light bulbs, Rectifiers in my ZW, and many mechanical e-units in my post war 2055.
As a whole, I find the original TMCC system rather robust, though I have far less experience than many others here.
JGL