As already mentioned, anyone not getting full speed its more than likely their end that is the problem, nothing actually wrong with the service. I wouldn't say "multiple threads" is a LOT of people, considering how widespread these services are now, that's still going to be a minority.
Personally I keep my Plex server at home, so having 110Mbit upload would be extremely useful for me to access it when out of the house (assuming its ever safe to do that again).
I'm not doubting that 500Mbit is good enough for your situation, but I don't think these reports of speed issues should play any part in your decision making process because as already pointed out, its almost guaranteed to be a misunderstanding from the end user of how the service works, a bad router or trying to get full speed over WiFi. There are people on this forum a lot more knowledgeable about how all this works.