Ubiquity stuff has always been flaky with metric accuracy, its commonly mentioned on their forums, and I get the impression they never intended it to be super accurate, just a general overview.
My personal experience with a USG is that under real load it will deprioritize stats, so traffic speeds etc start getting dropped, though I guess thats better than losing network speed just to make a pretty graph
Its true its aging but still works well for a small network. From what I gather in the forums, it might be due to the controller software not so much the specific hardware; the complaints still seem fairly steady.
For example the past year someone reported their printer used less than 30KB but was recorded as 1.6GB; another with a Cloud Gateway (not too old) reporting 17GB of traffic daily for a Kindle consuming less than 1MB
Edit: apparently Ubiquiti quietly EOL'd the USG, so no more security updates. Just bought a UCG Ultra, so far seems a decent replacement, I like that the little screen rotates when mounted upside down