I have been a proponent of 123 both because of its electronic technology, and because you get a brand new distributor to replace an old one with worn out bearings and an advance curve 'way out of spec. I have probably installed a dozen of them in cars ranging from a 1953 300s to Pontons to 280sls.
I have encountered two failures. In both situations, the engine would simply not "rev" beyond idle speed. One was in a 250sl and it failed shortly after installation. The second was a 2012 model from a 230sl that Gernold was working on. 123 tech support claimed that could not possibly be caused by the 123 but because I am a 123 "dealer," I was able to get 123USA to install new circuit boards in both and that fixed the problem.
123 has also advised me to ground the units using the tapped 6 mm hole in the bottom of the distributor. This has not yet made its way to the installation instructions.
I had one other suspicious failure. A 190sl with a 123 that I installed with a Bosch blue coil ran flawlessly for a couple of years and suddenly would not start due to no spark. I did all the usual checks, and finally sent the 123 to 123USA. They tested it and could not find a problem. Upon its return, I installed it with the additional ground, but still no start. Out of desperation, I tested the coil wire and found it had 5k ohms resistance. I replaced it with a zero ohm wire and the car started instantly and has been running perfectly for six months. Are 123s more sensitive to ignition resistance as they age?
Cheers,
CT