All games DLC and patches are thoroughly tested. Even patches that come out in beta testing first are well tested. However internal testing have a lot of problems first off 20-50 people testing a game internally will not come across as as many bugs as 100 000 people would in the same time, even if they are professionals. Now im just speculating here but i assume that they have computers with similar performance which could make the process even harder. Then after that there is the problem of finding the cause of the bug and correcting it without messing up the rest of the game.
As an MPC player you learn this quick as stability in the game varies from patch to patch, because CA often sacrifice multiplayer campaign stability in favour of singleplayer content... So it sucks for me that they care about you whiners, im just grateful if the game runs without desyncs.