Things can be difficult for wildly different reasons. Don’t let that surprise you.
I solved a 67×41 crossword puzzle in the last few days. It’s huge. There were 782 distinct clues and it took up 3 entire newspaper pages.
It took awhile, but the truth is, it wasn’t that difficult. Finding the right clue on the full page of clues was harder than determining the right answer! I have solved tiny 11×11 crosswords with more difficult clues; I’ve probably spent just as long on puzzles less than 10% the size of this one.
That isn’t to say that this one wasn’t difficult. It just wasn’t difficult in the usual way.
Some pieces are tough because of the harmonies. Some are tough because of the range. Others are tough because of the stamina it takes to sing them, or the lyrics, or the tempo, or the way the tenor and alto parts disagree.
If you’re only looking for one type of difficult, you might miss that a piece is difficult at all: if it’s tough in an unexpected way, you can think it’s easier than it is. I’ve worked on unison pieces that are more challenging than some pieces with divisi, and some 2-minute pieces that challenge singers more than some 6-minute pieces.
Assess why something is difficult or why it’s easy as you begin work on it. It’ll make the process easier if it’s not unexpected.