What’s the chord with the highest risk of making young singers struggle?
I would argue that when you move past triadic harmony, your highest risk is a major 7 chord with the root in the bass voice. Fighting against the major-seventh interval in a higher voice is asking for the basses to sing a half-step lower, creating an octave.
It’s doubly challenging if the melody is on the seventh, but it’s true in any part. In addition to it being hard to tune, the mistake also turns a major I-type chord into minor iii-type chord. Yuck.
What’s the solution? As an arranger, I think twice about voicing those chords in root position – basses will have an easier time on the third or fifth, and inner parts will adapt more quickly to the dissonance As a conductor, I drive home singing the part as a solo, as if it’s the melody. I want it so ingrained that they think that is the melody to Misty or Over The Rainbow!