Slave Pioneers
Magnar has a rather broken ability to make legions of slave pioneers costing no maintenance and 1 production. At least there is the minimum 3 season training period (at least, I'm guessing that's why it takes 3 turns to produce something that takes 1 production). Still the rate of early game expansion this allows, even with a 100% focus on grain over materials, is completely messed up.

I'm a little surprised I couldn't find any previous mention of this problem, as cheap pioneers was the first thing I thought of when I started a Magnar game (but not that cheap). I guess maybe it's still a relatively unknown trick that you can edit pioneers.
I suspect I'll find the magnar free slave unit ability to be a bit broken anyway. Not because they are militarily useful, but because they extend your visual range (there's no sneaking up on your borders) and also as an expendable distraction to delay AI controlled invading armies. But we'll see...