Based on the current values, presuming each shot succeeds with 1 success, it would take ONE-HUNDRED AND FIFTY SHOTS to take down an AT-ST with a missile tube (not counting crits).
Obviously not necessarily the number of shots it would take to take down an AT-ST in practice in the game, but I still think they are underpowered.
My fix suggestions: Increase Breach rating from 1 to 3 and/or change it to Planetary scale damage when targeting Planetary scale targets. This would mean that against an AT-ST a shot with 3 net success would do 5 damage (20 divided by 10=2+3=5) whereas against a trooper it would do 23 damage (20+3=23). this way it isn't a one-shot against AT-STs and at the same time it isn't going to necessarily put a PC or advanced opponent at double threshold immediately. (i.e. your Human Commando has Brawn 4 + Toughened 3, he has a wound threshold of 20, if he gets hit with one success, he takes 21 damage, meaning he is fairly easily healed) The reasoning behind scaling additional damage is that a Human is going to be a lot harder to score a direct hit against than an AT-ST would be, so the damage does not scale as dramatically because it is probably just a matter of getting the detonation closer to the Human rather than where on the Human you hit (because if you hit a Human in the chest with an RPG, you probably won't have enough left to bury). However, I can certainly see an argument for just operating it on planetary scale with no conditional damage modifiers.
What are some of your suggestions?