In the past few months I have bought and tried the Mini Groundhogs, the 8'' Easton Nanos, and the 6'' Easton Nanos.
The heads of 2 out of 4 Mini Groundhogs broke off when knocking them in after just a few uses. I never felt like I was pounding that hard on them nor was the soil extremely hard. I'm also fairly confident that my rock strikes were pretty in-line with the stakes themselves. While their hold is decent, I've deemed them too weak to be reliably used.
The 8'' Easton Nanos I got were junk...I bought a pack of 4 about 3 weeks ago and was able to pull the heads off of 3 of them just by using my hands. I don't think I would trust these not to break in half when it counts.
The 6'' Easton Nanos have performed okay for me, although I've only been able to use them half a dozen times or so. I bought 8 of these and, oddly, none of their heads have popped off yet. I only use these for non-critical, low-stress tie-outs. The weight of these is on par with what Easton specifies.
So maybe the 8'' Easton Nanos I received were defected, but it still doesn't inspire much confidence in my 6'' Nanos. I'll be sticking with my Groundhogs for key tie-outs and probably replacing my 6'' Nanos with something else soon.