How balanced are levels in 5e? I noticed a Fighter gains an extra attack at level 11, which to me sounds huge. Would a fighter therefore be underpowered at level 10 compared to other classes, or would it be overpowered at level 11?
Fighters are better as multiclasses up to levels 3~6. They get feats, extra attack and action surge (possibly best feature in the game) very early, but are underpowered in later levels. Also Champion might be the worst subclass by a long margin.