First off yes, a 180 ED discriminating circuit will accept everything which means more nails in the lower end that a 120 ED circuit can't/won't.
This is kind of the definition of the difference between the two...how much of that 180 degree range these different units can sense.
Even my Vaquero at about 165 ED would be silent over a few low end rusty iron targets my 180 ED Compadre would pick up easily.
Second, you never mentioned how much disc you were using if any at all.
Plus the Eldorado is a bit more powerful than the Golden so were these problem targets deeper than where the Golden was getting them?
Using my Vaq, a legacy unit related closely to that Eldorado, even rusty nails higher up the spectrum came in pretty nicely a lot of times but there were ways to tell what they were compared to better targets if you knew those techniques.
I hunt in an area of the country where we have mineralized soil and an insane amount of other iron pieces in the soil from tiny to large.
Surprisingly using my F70 with all metal or disc really low on 1 or 0 is actually more helpful to me in figuring out things more accurately here than higher disc does.
I also hunt with both a Compadre and now a Mojave more often than not with the disc really low into iron if not all the way down.
I have two friends I hunt with also and they both use AT Pros, one just got a Max and the other also hunts with a DEUS and both of them figured out eventually that hunting here listening to and getting signals on everything with the lowest disc possible on all these detectors is the best way to be successful...here.
We still dig iron here but not as much as you might think because we learned how to avoid tons of it despite that low disc.
It just takes time to figure these things out.
Higher disc can be a friend but sometimes lower disc, and some good basic knowledge and experience, can be a better friend to you around heavy trash and iron.
Iron can have a slightly different behavior than good targets do, even those that come in higher, but you need to learn to understand how.
Sounds like the Eldorado is working fine with the data and behavior you have described, you just need to get a handle on it a bit more maybe and adjust the disc better and a little higher or get a bit more knowledge about how actual iron behaves on this one.
Some more precise info about your settings and what is happening can help us help you better.
For me I much prefer using 180 ED units than 120 in my sites...but that is me.