In the ‘Curse of the Blood Elves’ campaign, Kael’s Siphon Mana cannot be used to transfer mana to friendly units. When attempting to do so, a tooltip pops up saying that I must target an enemy unit, despite the description saying that friendly units can be targeted.
Additionally, Kael’s Siphon Mana does not seem to be able to temporarily raise his mana above the maximum limit as the ability’s description mentions.