I was running with a stock MP5, and never had any problem with it, even in outdoor. It's upgradable to hell and back, so there's isn't much you can't do with it. The folding stock is nice for CQC as well. There are only a few issues I've had with it, most of which are fairly minor.
1) The scope mount for the RDS gets in the way of the iron sights, so you won't be able to use them both en meme temps. Either get a different mount or don't use iron sights. I personally opted for door #2
2) A common problem with all mp5's (AK's and a few other rifles) is that the front sight/flashhider will fall off if the screw on the bottom isn't tightened. Again, an easy fix - give it a quarter turn with an allen key every few weeks or after a day of heavy gaming (If you don't know what I'm talking about, you will once you see it.).
3) For outdoor gaming especially, a mini battery isn't going to last too long. There are a few options here. You can either carry multiple batteries, get a high mAh battery, or throw a battery in the stock.
If you're really unlucky your mechbox might crack, but that's a fault of the v2 box rather than the mp5 design.
All in all, the mp5 is a good choice.