I've found the F5's depth to be very good, and I like the detector a lot. But it has it's quirks.
First of all, if you are using 3 or 4 tones, don't expect to get a solid two way high tone hit past 5 or 6 inches on a coin size object. Beyond that depth the ID will get jumpy, but I've still gotten a reasonably solid tone down to eight inches on coins. But not a high tone, sometimes a hint of high tone mixed in with a lower tone.
Secondly, if I run my unit stable with no chips or chirps when I'm not swinging it, I can't run it hot. In fact I would have to run it cold - something like a gain of 30 to 40 with a negative threshold. Obviously with that kind of set-up there wouldn't be much depth.
The key here is I can run it hot (90 gain and 0 threshold , 50 gain and +5 threshold, or somewhere in between) and in my moderate soil it will be stable WHEN I'M SWINGING THE COIL. When I stop swinging the coil, it will start chattering. This is one of the quirks of the F5, or at least my F5.
Do a search of Mike Hillis' posts for his F5 manual revisions. That will definitely help shorten your learning curve if you have the original mistake-laden manual. Ground balancing is simple as can be, but the manual (at least the original one) explained it totally wrong. Bill Ladd's video shows how to do it right.