How so? CSS really isn't hard to learn and/or play, it's just the super-competitive freaks who play in their mom's basement all day that make it remotely difficult...
What makes it awkward is the recoil is very difficult to get used to. When stood still, your first bullet will go exactly where your crosshair is, so a great first headshot means an instant kill. Your seond bullet will be kind of accurate, and your third could go anywhere. When your moving it will also wreak havoc with even your first bullet. It may go straight, it may not, but like it real life any big movement affects your accuracy.
This means if you happen to miss your first shot, then all of a sudden your on the back foot, cause your recoil is no longer under control and most likely your opponent has the oppurtunity to go for a shot himself. So CSS is largely burst fire 2 shots step aside to avoid their shot, burst fire 2 more, etc etc. However if you time it wrong you give your opponent a nice moment where you are stood still for him to score the headshot.
Bottom line spray most of the time simply doesn't work, and body shots may take 4 or even 5 shots to drop a target. You may get the drop on someone, but if you go for the body and not the head, you give them precious seconds where a single bullet can instantly take your head off.
Because it is so easy to get killed its very much a game of guessing where people will be, and trying to have the shot lined up ready to fire when you see them. You guess wrong, you die.
Thats how I see CSS anyway. Having played CoD4 before CS:S I have major respect for a game thats 5 years older and still is better in every way to its modern rival.