One way to realistically rebalance the missiles would be to either drastically decrease the scout ufo's hit-points or to increase Avalanche's damage so that it would always destroy scouts.
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I dislike the real-life comparisons, because you know, it's a game. I just wanted balanced game play.
Decreasing scout UFO HP is actually opposite to my own private ruleset, which increases HP for all ufo craft and range/damage for some, mostly the smaller ones. Shooting down small UFOs is almost too easy. I wish small UFOs had an "evasion" penalty to your weapon accuracy, to make it not just a "pew, pew, pew, crash" every time.