A modern suppressor won't affect accuracy in any way other than velocity, most modern suppressors actually increase velocity a little bit, like 30 fps. I shoot suppressed at very long distances and haven't noticed any difference.
The only disadvantages I can think of, is
A) It adds weight to the front of the gun, the m4-2000 weighs like 20oz
B) It adds length to the gun.
Personally, I'm a big fan of suppressors, the weight doesn't bother me much, but the length length can be an issue. They make a 10 inch barrel like a 16 inch, a 16 inch like a 22 inch. I think that the trade off is worth it in most circumstances. I really like the shorter suppressors for my ar-15s and other guns that are designed for "up close".