The Indian Head Gold coins, Peace Dollars, I'll have to agree with. Those are some of the toughest coins for me to grade.
If any park of the middle most part of President Franklin's head is even somewhat bald, it most likely would not hit an MS grade unless it was originally a very strong, elevated strike.
To help get over that problem you first have to realize that no two types of coins have the same kind of luster. What I mean by that is this - there are many different kinds of luster and no two of them have the same look. Morgans, Peace, Walkers, Frankies, Wahingtons, Jeffs, Seated - they all have different kinds of luster. A completely different look than the any of the other coins have. That's why you have to become experienced with each different type of coin in order to grade them accurately. You can't look at a Peace dollar or a Frankie and be thinking about the luster on a Morgan when you grade them, or you will get the grade wrong. To be able to grade any series with any hope of consistent accuracy you first have to have examined hundreds of thousands of those particular coins. That's why it has always been said that anybody can learn to grade if they have two things - a good book on grading and at least 20 years of experience. Minor correction - he was never President.
I think that's the key. So, everybody please send me your incuse quarter-eagles -- I need the practice!