Played a few TDM matches in the Alleys and there are definitely a few spots where your team can get decimated. In the two back corners there are dead-ends large enough for your team to spawn into but all exits are easily defended by the opposing team (they get to cover all the corners and they get the roofs). If your team isn't working together to punch through then your only hope is that the other team will wipe you all out before you respawn. As it was I spent about five minutes spawning into the alley and getting killed. Out of the top 20 players at games end only 3 of them were from my team. It was pretty frustrating, but at the same time if our team had been working together I think it would have been manageable. Once a few people broke through we started spawning elsewhere.
The moral of the story: You will generally spawn into a group during TDM so that you are never far from action, but it is a good idea to keep a small strike force removed from your main army so that you can maintain a mobile spawn point. If you don't do this then you do run the risk of getting backed into a corner.
As for the Marksman, it is a good weapon, but it seems a bit overpowered for my tastes. The tracking on it is pretty strong so that you get a quick and easy lock even when you don't use the scope. True, headshots are where the real damage comes from, but from mid-far range the thing is a bit too effective. You can snipe all the way down the alleys, or in Orick pretty much straight across the map, and its lock on means you don't have to be as precise as the fareye to get kills.
I personally prefer the carbine (I think it is more lethal at mid-near range then the first game) and love bouncing from roof to roof in Chicago and bombing snipers/marksmen with the 40mm
Other than my gripe with the Marksman, I think all of the weapons are very well balanced. The shotgun is a double-barrel then melee killing machine, which in my mind makes it a great close range weapon that offers the opposition a bit of room to retaliate. In R:FOM if someone got close with the shotgun it was over (so long as they had even a modicum of skill).