Don't get me wrong, they are awesome models (just got my box) but nothing to be scared of. They are like the most glass cannon unit I've ever seen. A single Tgheist scream will blow a hole in them, next to their Ld7 general or not. Unfortunately any unit is a bad choice, as they will turn and blow them away with shooting. Lots of magic will blow them out - Gaze (or any magic missile), raise dead to continually waste their shooting, death snipes from a necro in massive zombie buffer. If they want to go all ratling, one shadow wiz can neuter them all game with miasma.
Also, To allow 3 ratling cannons you're gonna have to punch WYSIWYG in the gut because not many people are buying 3+ boxes just to go all rattling.
3 of them in a unit will take the role of a Abomb in the skaven army:
Anvil
While they don't have regen, they have twice as many wounds. At 4T they are not as tough so Abomb is a better anvil. Versus fire the SF would fare better, but only becasue they have more wounds to soak up the damage. A couple fireballs would probably do equal hurt on both units, but the SF would lose at least a model and have to flee on 7Ld. Against mass shooting, SF would do great, as they can claim bonuses from walls (there not Large Target) and have -1 due to skirmishers.
Hammer
With strength as much as 7, a CC unit of SF will definitely provide a more reliable wound output, but if they go all rattling a lot of close combats would be bad for them.
Shooting
Obviously impressive shooting. Abomb is still great for catching things like fast cav though since its got crazy random movement that allows it to "charge" from any direction.
SF versus Abomb.
Even if they didn't move, 3 ratling will have an average of 33 shots at 4+ is 17. At 4+ to wound thats 9, and at 4+ Regen thats 5 wounds. On average, an Abomb could stand a full barrage of perfect shots from 3 rattling cannons. But an Abomb can charge without provoking a stand and shoot. The impact hits are wounding on 3+. The SF get to attack first but they first have to pass a terror test on 7Ld. Regardless they wont kill an Abomb, and whatever an Abomb rolls is going to hurt them bad. Where a few wounds isn't going to bother the Abomb, its going to remove models for the SF therefore in the long run, Abomb will likely win. Also the abomb can come back to life just for fun.
Obviously the SF have merrit over an Abomb - powerful shooting is a big deal. I'd much rather have some SF against Wood Elves. Skaven ballistic shooting is actually relatively absent - you dont often see heroes with guns or the gun teams. Most of the shooting revolves around the WLC, which are actually the least accurate cannons in the game - despite being able to shoot through walls, a WLC will not cause the type of guaranteed destruction that standard cannons can. A doomwheel can be fairly easily controlled through smart deployment and use of terrain. Other than that you dont see much shooting, so some shooty SF actually changes up the army quite a bit.
SF also create a new dynamic for skaven - I find frequent opponents bend their points to have great fire options for the Abomb. Now I can show up with some SF instead, so their points in fire are wasted. Your opponent will have no idea whats going to happen... an important change for Skaven who have very limited variation in their lists right now. Now we can show up with double warpfire that appear behind the battleline on turn 2, or a super hammer thats going to punish nearly anything in close combat, etc. Everyone is excited about the rattling rules but I think there a lot of fun diversity to play with here.