A lot of the problems with MTA are due to the fact that it is client-side. When you shoot someone on CS, the server notes that they've been hit without the client's intervention. With MTA, the client notifies the server that the player's been hit. This makes the game more susceptible to lag. It also makes it easier to cheat because you can prevent the packet saying that you've been hit from reaching the server. It also explains why people can pause to avoid being hit.
The problem is that they're hacking a game that's not designed for multiplayer. If it was server-side, the server would need to know a LOT more about GTA. At the moment, it basically relays packets and keeps score.
It's a beta. The idea is that the public are testing it. MTA's been in development for over 2 years now, and I'm glad that they released imperfect versions rather than keeping us all waiting until the final release.
http://www.mtablue.info