Competitive fluffiness and Notes about ETC
ETC is the European team championship, they play using a special set of rules that basically cap all the power armies (including us) in an effort to "balance" the game. Having said that the ETC restrictions on Skaven are:
• Rat Swarm units, Giant Rat units, Warlock Engineers, max 3 each
• Skavenslaves, max 3 units and 120 models
• Warp-Lightning Cannon/Hellpit Abomination (max 1)/Screaming Bell, max 2
• Up to 2 units of Gutter Runners/Storm banner/2nd Doomwheel, max 2
• Doom Rocket (30)/Brass Orb/Power Scroll, max 1
Fluff armies are a great and can spawn some cool ideas and some amazing games, but they are not as competitive as normal lists simply because they handicap themselves. If you want to read an excellent article on writing fluff lists that are friendly and fun to play I highly suggest you check out this guide here
Simply put, Fluff is fun. But it doesn't win games.
Currently the most competitive fluff armies are themed around pestilence. While these armies can be competitive they are still by no means equal to the power a normal list puts out.