I've now built a rifle and a pistol using EP80 lowers. They're great for lightweight builds and pistol ARs.
Things I've learned and tips:
1) Take your time, especially when drilling the safety selector.
2) Drill the grip hole all the way thru, then tap it.
3) Squeeze the bolt hold open pin down and smooth the edges, so it won't buldge the polymer so bad when it's inserted. ( I've had to do the same thing to $350.00 billet lowers)
4) 'Metal' epoxy putty can be used to plug a mis-drilled holes. It can also be used to snug up a loose safety selector hole.
5) The only Dremel bit you need to finish the lower is
THIS
6) Don't freakin' sand it unless you want it to look like crap. The Dremel bit listed above will actually leave a decent finish.
My 300 Blackout / EP80 pistol.