Okay, I'll take a shot at giving you a direct answer to your question. First off, because the "shudder" is not deemed to be a recall, there is no published statistical data that would show the true rate of failure. Anecdotally, the perceived failure rate is believed to be between 2 and 5 percent with the majority of affected C7's coming from the 2015 and 2016 model years. There have been a number of component modifications and improvements, valve modulator bodies, torque converters and ATF's, since the 2016 MY that have helped to improve the problem. Any vehicle produced after February, 2019 has the latest ATF and the most up to date transmission components.
Just a few points of clarification on this issue. First off, the torque converter "shudder", as related to the 8L90 8 speed automatic transmission, is caused by the operation of the Active Fuel Management System (AFM). The problem occurs because the lock-up clutch in the torque converter fails to lock completely during the change from V8 to V4 mode. This is now thought to be due to excessive moisture absorption by the ATF within the transmission, or so says the latest from GM. They have gone through 3 different formulations of Mobil 1 Synthetic ATF with the third, and so far final version, being the most successful. The general consensus is that the latest triple flush approach with the newest ATF (see attached TSB) will, in most cases, solve the problem.
I have put 15,000 trouble free KM's in almost 3 years on my 2019 GS with the A8 transmission, although that may not be sufficient mileage in order for the problem to appear. Many owners do not experience the "shudder" until 30 to 50 thousand KM's or more. But consider that more than 95% of C7 owners, with A8's, have never experienced the problem and probably never will. Did GM drop the ball in terms of customer support with this problem? Yes. They, in my opinion, should have recalled all vehicles with the 8L90 transmission and fixed the problem, once they had an effective resolution, but that's not happening. Can the problem be corrected? Yes, for the most part, with the latest triple flush that will be performed free of charge under GM's 5 year / 100,000 kl powertrain warranty. Would all of this stop me from buying a C7 with the A8 transmission? Not a chance, love the manual mode of this transmission with it's quick, precise shifts ( in Sport / Track mode ) and the ease of use as an automatic, for the wife of course.
Hope this helps.
Best regards
Ross