The only things I'd be worried about are the captain's chairs on the Signature model, if they happen to operate the same way that they do in Japan's CX-8. Infotainment shouldn't be an issue since they just made the display larger, but who knows - in the CX-9's first year or two of production, there were some infotainment issues that led to poor reliability scores, but all of those issues should have been ironed out by now (my 2018 has worked very well since I took delivery). No idea if changing the screen size will affect how the current system performs.
Some other things that a few 2018 and 2019 owners have reported are wheel/tire vibrations and the occasional front suspension clunk. These are usually handled well by dealers, but there are a couple of exceptions where people had to deal with poor service at the dealer. I have a 2018 Signature with almost 32k kms on it, and I have nothing but good things to say about mine. I did an oil analysis recently and according to the lab, everything was normal. No odd interior noises aside from those made by my daughter's car seat. Mechanical reliability is solid IMO, but make sure you take the car for a lengthy test drive and test all features to make sure you know what you're buying. For example, a few people bought the car with the assumption that it has Lane Centering, which it does not. Some others find the seats uncomfortable (largely dependent on they way your body is built).