Hello there,
Stormboyz aren't totally bad in a way. They are good in rushing unexpectedly towards enemy shooters, or pounce unto advancing units to block them from your lootas or other precious units. Most people don't like them cause they are difficult to use, requires a lot of finesse to position them in cover and timing to pounce on the enemy.
Most people think they are bad because rushing with them is totally suicide, and the Stormboyz get less bang for the buck after the Run rule is implemented in 5th Ed. People will rather use more regular boyz to run towards the enemy.
Zagstrukk is like a "hit or miss" thingy about his deepstrike and his charging Power Klaw, somewhat similar like the Wierdboy/Warphead, Flash Gitz, Zzap Gunz, etc. In short, they are all too random and inconsistant. Plus they take up valuable Fast Attack slot, and there are more Fast Attack Ork units which are cheaper, less random and equally as dangerous, such as TL Rokkit Deffkoptas or Buggies/Trakks. Bikers are also tougher than the Stormboyz due to their natural cover save, increased toughness, and bigger dakkagun.
But if played in a friendly, non-competitive environtment, Stormboys are good. Zagstrukk and the new models look too.
