With GM it was a couple of different problems from 2020ish through current. They released a new valve body with a hardware change in 2025-2026 to try to mitigate rear wheel lockup, but a lot of the other issues are still there. Cheaper metals/bore wear etc. over time. The number of vehicles effected was extremely high last year, people waiting for months on their truck while valve bodies were on backorder...it was a mess. There is no way for the general public to know the exact number that are affected, however you can join the facebook groups for GM products/GM rear wheel lockup to get an idea of the volume, it's not small. Back in 2024, before many of these transmission were failing, GM noted they already had 1,888 warranty claims JUST for the rear wheel lockup issue, that doesn't include all of the other issues (dead transmissions and valve bodies).

Here's more technical detail from NextGen on the problem and how they fixed it: https://nextgendiesel.com/blogs/transmissions-101/general-motors-10-speed-transmission-recall-here-s-how-we-fixed-it

Some people don't have problems, people that perform maintenance every 30k probably have a better chance at longevity, but some of it is just up to chance with the quality of the product you got on purchase, a bit like roulette.

Ford seemed to have fewer issues but a lot of the perception was because they had parts on hand and generally fixed vehicles without customers waiting months. Ford didn't have the rear wheel lockup severity, and they worked through their issues with the CDF drum by I believe 2023, from then on they should be more reliable provided proper maintenance is performed.