[Micah]

You sum up the advantage of a sneak attack pretty well, but I don't see how that relates to gem items.

If you lose 90% of your territory in a turn and can't reclaim it quickly you're sunk either way...there's still a large gem swing in favor of the attacker and your income will drop below your upkeep. In either situation you've got to rely on what you have on-hand to orchestrate your counterattack, not what your income is providing. Once the attack is sprung the difference between having 20 gem income versus your opponent's 80 without gens and having 70 versus your opponent's 130 with them is of little importance, you're still way behind and unlikely to catch up, even if the proportions are comparatively better. At that point the defender is reduced to protecting the few castles they have left, since an offensive push against a superior opponent that is expecting it is folly. Which leads us back to extreme cases of turtling, which it seems no one likes.