UV breakdown is not common with Atomics (unlike some Oceanics and some Zeagles, and the first generation Legends for that matter) but i see them from time to time.
There are some brands/models I work on that i have never seen an issue with.
Here are a couple of examples of Atomics that customers left behind when they bought a different reg. On the left, the purge cover disintegrated (the gray dust below) and on the right, the rubber trim peeled off the adjustment knob.
I dont want to bash Atomic -- they are fine regs and i would imagine they have corrected the issue -- but the OP asked for opinions and i am giving my observations.
Not doubting what you said but it may have been an uncommon issue or something that occurred for a brief period of time. I have several Atomic regulators and I have used them in the Middle East's brutal sun (UAE, Libya, Egypt) and never had an issue. I dive mostly from shore and I have to swim for a long distance usually to get to the drop point so my regulators, and other equipment are exposed to the sun more often in addition to sitting on the beach before and in between dives. The oldest Atomic regulator I use frequently is from early 2000's and the newest was bought in 2012 (I think). The newer Atomics bought after 2012 aren't used as much, mainly for training, but still used frequently enough in Libya's sun.