I think the variation in tracking accuracy you mentioned is probably normal and should be expected with all such devices. Frankly, I consider their accuracy (10m to 30m) is actually adequate.
I agree that they won't be totally accurate and wouldn't expect them to be. Even a high end dedicated GPS unit can be unaccurate at times. I've seen a few posts online comparing the Fenix 3 to the Ambit3 in tree cover and the Ambit3 wins every time. All posts I've seen seem to be a few years old though. I was just hoping that Garmin has worked out the bugs in the new units.