Ok, maybe I was a little blunt in my post, not leaving much description, but to claim it's flat out not true or wrong is a bit dismissive.
I had a few of the issues described early on my GT5 online play. Invisible cars, I myself was an invisible car and ruined a few players races. Didn't matter the size of the room, there was generally a few cases of it that occurred more often than I liked in online play. Not to mention disconnects and not being able to connect to rooms.
In my case, NAT 2 was showing and UPnP was enabled. I moved my PS3 into the DMZ and it fixed all my issues. (I had originally set it up like that but forgot to adjust it when I changed a few settings months prior) My other PS3 is in the same boat, but I use Port Forwarding rather than DMZ, that also works fine and is a more secure option than DMZ.
Since getting my modem/router setup correct, I have not had any issues, not a disconnect, not an invisible opponent, no issue connecting to rooms etc. So rather than flat out dismissing it being a settings issue, I suggest that you check your end, make sure your settings are correct and you have done everything in your power to make sure you have a well setup network.
At least this way you know any issues are outside of your control. Unfortunately there will always be someone who can't connect or can't play cleanly online, this is just the nature of the network with different hardware, ISP's and settings. Just don't take the theory that if game X works online, game Y should work the same.