The Duramax and Allison are also medium duty and is Ford's Powertrain. Not sure why that drum keeps getting beat, but it does at least by one poster.
As to the reliablity question. My newest Duramax 3500's have been a '15 and currently an 18'. I also had a '15 3500 with the 6.0. I have had numerous other Dmax trucks but if you buying new this likely of the most interest to the OP. I work these trucks, pulling equipment during the week and a car trailer or Toyhauler on the weekends. I have had a computer update and a DEF sensor on the 18'. The 15' never saw the a service center except to get the free oil changes that GM stopped doing unfortunately. I also have a 19' 3500 that the business owns. It only has 10K on it. There is an issue on that truck with the theft deterent system that I need to have looked at. I am really happy with these trucks. If I could change one thing, it would be the dead petal on take off. If 2020 is nanny free as they say, there should be no dead pedal on take off. I would be all over that. As far as reliability, I have nothing to complain about and I have had 2 Duramax trucks at a time since 01. If they were unreliable I would not drive buy them.
Lastly, as I have mentioned before, I really dont see a need to avoid a 2020 because it is a first year of a model change. I have had an 01, and 08, a '15 and this 18'. All of these trucks were first year or second year of a new version of a D max the truck itself, and in the case of the 01, first year of the Dmax era. I did buy an extended warranty on the 01, and just before I traded it in at 100K the computer went out. My cost was exactly what I spent on the extended warranty, turned out to be a wash. Never bothered again and have never been bitten. The 2020 is ready for prime time and if there are some issues that come up, you have the factory warranty. If something really crazy shows up, GM has been good about extending the warranty on the component in question. I would not let the new HD being a first year truck get in the way of buying one if that is what you want.