I'll add to what the fellows said above.
Most (all?) machines go nuts around very strong electrical interference and there's nothing that can be done other than turn the sensitivity down so low you'd probably do better with a shovel and dirt sifter.
Some brands have electrical interference circuitry built in that basically does the same thing as turning down the sensitivity. Either way depth is lost in areas of extreme electrical interference.
Now, on the other hand, some areas have moderate electrical interference and they are workable with a good machine. In my book, this type of problem is what separates the truly advanced quality machines form the garden variety brands.
If a detector gets decent depth with a reasonably smooth signal at a site where most others can't even function, you have a truly quality built machine. The only way to know for sure about any machine is to try it. Cost has little to do with real quality today.
Badger