Low iron areas with a thick top soil (how deep to clay line) a large dd can do well. So there are a few areas that are wide open here where targets are not concentrated and have a top soil well beyond 12". My larger dd's do ok there.

High iron infestation and don't want people to confuse that with natural iron that can be generally ignored with ground balancing, the only choice is an analog machine with threshold based discrimination with a concentric coil. These in my experience are the only machine/coil combination that can reveal a higher conductor below nail or piece of rusty steel/sheet metal.

I have quite a few dd coils, mainly because there tends not to be a lot of options for the machines sold. DD coils seem to have a very directional signal that seems to reflect on flat targets giving an inaccurate response so washers, bottle caps can have unusually high phase shift and silver can even iron wrap. Something that concentrics don't do.

Trend on machines is faster signal response and more complex look up tables so people can hear pseudo nuances. The trends seem to be splitting hairs and trying to snipe out targets that are close to trash or iron but all that is just superficial as DD's are very top down and mask too easily. All the new tech plastered over the analog core of machines is pretty cool but I don't see how they can map out the infinite variables we see in the real world.

I like hunting targets and developing a trust in the audio of my Tesoro as it is rarely wrong. Chasing numbers can be exhausting. The absolute tranquility of the beep and dig is incredible when your just walking over the iron trash and only hearing possibly good targets.

GB, don't be afraid to kick that discriminator way up. Vaquero shouldn't disc out a silver dime if you disc out a can. Those 8x9 coils though not incredible deep seekers can pick a dime at 9" or so depending on mineraliztion. That is still 9" or so more than any dd coil with iron in the area.