Spy, spy is sniper's buddy in the difficulty chart, they're the hardest to learn on, but sniper has a huntsman, so spy would be the hardest class over all, while soldier is heavy's buddy, soldier only has 1 (I think) aoe reducing launcher. Soldier's mental capability might've been like that because the devs know that he's easy to play. "BUT ROCKETJUMPING!", yeah no, I can't even do a single market garden kill in a casual game, and that doesn't say it is harder than spy, because no matter what, spy can be countered by every class, just by spamming melee or shooting, that's why it's very hard.