All of the following is with respect to Copperplate/Engrossers script.
It used to be any majuscule. :-)
I read a paper on Iampeth by Joe Vitolo on the basis of all letters on the oval, and suddenly a lot made sense, and most letters got better.
Now, I have the most difficulty with anything with a long upstroke connecting two downstrokes: M, N, V, W (ok, V has one downstroke...it is still hard for me).
And then is 'z', both the minuscule and majuscule. I have trouble achieving the right slant to the weighted down strokes.