The broodlord and Necron lord bubble abilities can be used on multiple units in a given turn. The overlord ability 'my will be done' is one of the extremely few abilities that can only be used on a single unit a turn.
Aye, but their abilities are too circumstantial. Realistically the necron lord's reroll to my morale is almost never going to be a problem, as necron units tend to be small and resilient enough to either pass the test or they'll be wiped out anyway. And the broodlord is great with genestealers but after multiple games I've had just as much luck splitting them up.
Captains and lieutenants give a blanket reroll to attacks to all units nearby. I've felt firsthand how devastating that can be, especially when the bastards work together. Three squads laying on the hurt with rerolls, plus the guns on the characters themselves. Any of my Overlords can only buff a single unit each. What the standard lord offers just doesn't appeal compared to that.