In what way specifically?
Six axle locomotives (especially longer wheel base ones) require better quality trackwork. You can get away with absolutely awful track work with a 4 axle diesel.
Ive got a pair of BLI SD40-2s that run just fine, as well as a BLI E7. Only had a minor decoder glitch on the E7 (discussed at length on this forum somewhere, and yes I did get the new decoders installed, no I did not get to test them).
Im not a fan of having to move the locomotive for the sound to start though.
As far as Athearn goes, well, I had a Genesis GP7 show up (new from LHS) with a wheelset popped out of the side frame and detail parts floating around in the box. Athearn received a phone call and an email. Response was that other modelers were complaining about too much glue (except that in my case there was no evidence of glue). Also its weight is on the light side. My guess is this was a Monday morning or Friday afternoon locomotive.
Every manufacturer makes their own share of lemons.
I recently purchased 3 Intermountain SD40-2s, all from same production run, three different road numbers. 2 of them ran perfectly, one picked #6 turnout frogs. It had a warped truck upon close inspection (sent back for repair).