Angels appear stronger under buffs because they are inherently stronger. An angel with the Prayer buff has almost 300 HP, and if there's also a druid with high power in the army, that's an additional 25% to damage and HP, resulting in stats close to those of a Champion. Then, try to defeat angels protected by a Guardian with 5-6 charges.
And then, with the same set of champions.
By the way, if the terrain on the battlefield isn't favorable, their effectiveness will drop significantly.
Champions seem to have a speed of 26, while angels have either 30 or 35.

P.S. The War Drum is used in melee combat when its mana runs out.