Let me elaborate. Ravengaurd successors are a common enough element in competitive play these days, and this is what will unmake you. However, any marine army with scouts (and usually 3 thunderfire cannon) is EQUALLY fast at removing your screens.
30 gaurdsmen in 3 squads won't last long enough to affect the game on any level in this meta, and you should be thinking hard about how to structure a gaurd army to not use imperial gaurd infantry as screens before you go competitive.
Because in competitive play, gaurd troops is definitely paper to the marine scissors.
6 aggressor = 222 points. (108 average bolter shots ranging to back of your deployment zone/board) (-1 cp to infilitrate, and arrived nearby using "master of ambush" before the game
30 scouts = 354 points. (another 108 average bolter shots ranging to nearly the back of your deployment zone) (12 stormbolter in there. They start 9 inches beyond your zone and begin by firing._)
any ravengaurd officer (cheapest is a techmar) 45 points. (one officer is necessary to get the 6 aggressors to start 3 inches from your deployment zone without having moved. That is how ravengaurd says hello.) (-1 cp for master of ambush trait)
Getting all of this to shoot on the first turn at a gaurd's infantry before he moves is something almost any marine player can afford in competitive play, and scouts are not even the worst marine infantry that can do this scout move stuff. Non-ravengaurd marine armies tend to have multiple thunderfire cannon firing, and maybe a whirlwind, and some flyers.
Go look up the gun, and imagine 3 of them shooting 4 times at your screens (because one will doubletap) and all but 2 shots hitting you.
Then count scout bolter shots from 9 inches outside your deploy zone. If they must move, then count half as many from 3 inches outside it, or 21 inches into it.
Now figure how many gaurdsmen you need to buy to have any alive when you finally get to move, on turn 1. In the best case, I reckon its about 60 less than you had.
30 is the number the scouts alone kill (well;, 25, but then morale phase kicks in, and the other 5 die)
three thunderfire will average something like another 20 kills (24 hits that ignore cover, wound on 2's and have ap-2, its just "did you roll a 1 to hit, and was there a lieutenant nearby to buff this?), while the ravengaurd aggressors will pop 40 or so. For another cp, it becomes 28 or so more kills (and then, morale)
That is pretty much every game I play these days and about 2/3 my local competitive opponents run some varient of the 30 scouts, 3 thunderfire, or 30 scouts, 6 aggressor ravengaurd, or most horrifyingly, a ravengaurd alpha strike detachment in front of an iron gaurd indirect fire artillery column. Uncompetitive games? You may find yourself shooting at a pair of landraiders with 5 terminator in each one, or at a line of orcs.
Course, 2 landraiders will kill 30 gaurdsmen at range, in one turn, but they won't be there so fast.
Whew. Rant completed!
I hope this give some insight into why we are all saying "uh, your screens, maybe build up?"
edit. My main nemisis in this game uses centurion/assault instead of aggressors, because they can move in, ignore cover, and have 12 inch ranged flamers before the charges they throw.
Depending on deployment roll, this will simply end the game for my traditional gaurd.