I've played in multiple online dynasties and had 0 problems, they were awesome.
Mine was fine for the first season, at least as far as my buddy and I noticed. After that, it became unplayable. We'd play a game, only to not have it load. So the person would log off and back on and the information displayed correctly and we advanced the week, only to see the result change. The second season my buddy lost two games and a Heisman candidate to simulated games and I lost my bowl game due to incorrect simming. Then it got to the point where we couldn't even connect to play against each other. I followed the EA forums for a while and there were lots of people having the same issue. By the time a patch finally came out, we'd both given up.