Soil mineralization does have an impact. I participated in a test whereas a silver dime was buried at 4" and a mini ball was buried at 6" in some heavily mineralized Arkansas soil. This was in an unworked pasture. Even with the Cibola super tuned, you had to listen hard to pick up the signal in disc mode, the all metal mode got a bit stronger. The DeLeon was even worse and the Fisher 1236X2 got a piece of them in iron null mode. None have manual ground balance however the Fisher 1270 got a strong hit on both. Unfortunately I didn't have a Vaquero or Tejon to run on that test to see if their ground balance would fare any better.
The DeLeon did pick up several strong mini ball signals at 6"-8" in a plowed field about a 1/2 mile away. Since the wife was running the Cibola in that field w/o any signals, I'd have to say she didn't get any targets under her coil.
The Cibola is a great little machine for certain things..I believe it works better in no to little mineralized soil and is actually designed as the Tesoro's answer to a competition machine with the freq. shift, faster sweep speed and recovery time, and targets that are 6" or less. That's not to say that it won't go deeper.It's also a nice, very light, turn on and go machine that works well for parks, house lots, and dry beach hunting (with it's 14.3 to 14.7 kHz freq., it should be a bit more sensitive to small gold).
Just a few thoughts and opinions.