While I think they are a bit more reliable than the in the earliest years, the S door handles do still fail. I had one replaced (the whole thing because the service center didn't have parts then) in 2016. As it happens, the same handle failed again a couple of weeks ago — broken wire to one of the microswitches, perhaps the most common problem. In addition to replacing the microswitch harness, the mobile service technician replaced the paddle gear, although the old one showed no signs of failing. The cost for the parts, sales tax, and service visit to my garage was $237.
So, I've had one handle fail twice (front passenger side) but not the other three. Go figure.
If you want to buy a used Model S you might just budget for some repairs, assuming that you don't get one with a warranty. The big stuff, battery and drive unit, has an eight year warranty on most Model S versions. Prices are way lower than when I bought my used Model S; assuming that you get a low enough price you can afford to pay for some repairs if they crop up.