large minion armies have very high sustained damage while the minions are alive (or being resummoned fast enough). 

there's kind of a big disparity between Oak/Erebus (who can spawn large numbers of their own minions) and Sedna/Queen (who are maxed at 4 of their minions) when it comes to equipment benefits. obviously more minions compounds equipment benefits and Morale Skill point benefits by a greater amount. this is why you rarely ever see a Minion build of Sedna but its pretty common for Oak. 

Oak's Spirits in particular are so incredibly easy to spawn in large quantities and rapidly replace dead ones that he makes a good candidate for stacking minion damage gear. 

Ring of Divine Might (favor item), Gloves of Fell Darkur, and particularly the high end bonuses of artifacts (Unmaker, Bracelet of Rage, and Ashkandor all have great minion bonuses) can make this deal extreme damage. Oak makes it even better still with Surge of Faith which provides the sort of attack and run speed bonus that minions need to get their hits in against mobile demigods. Its quite lethal. takes a while to develop though. Oak is already a late-blooming kind of Demigod and the minion focused build in particular can feel underpowered until level 7 or even level 10 really. 

Oaks minions scale too good. Erebus minions are cheap but QoT and Sedna minions suck.

Scales really great the entire game on Oak and Erebus, but giants are a huge problem, your minions get splattered across the floor by them, like DeadlyShoe said.

I've tried the Ring of Divine Might out on a few maps with Erebus, and it make minion DPS pretty damn nice, but on cramped maps like Cataract and Zikurat with one to three towers on every corner I'd say the Horn of Battle is a much better choice because your minions can now attack the towers for 10 seconds without getting zaaped to death by the AoE in 3 or 4 hits, or DPS an enemy who thinks they're safe from your minions in tower range.