I was in a rehearsal once where the director told the choir, "This song is hard." That was interesting to me because I try to avoid using the word "hard" when I'm teaching a song.
I'll say, "This one is going to take some work," or, "This next part of the song is the most challenging part." But I think the word "hard" can be a turn-off to some choir members. If a song "is going to take some work", that suggests that they can definitely do it if they put the work into it. But if it's "hard", they may get the feeling that maybe it's possible or maybe it's not.
That's my thought, anyway. I'd be interested to hear how other choir directors feel about it.
Also I have a web page with my suggestions for how to approach teaching difficult music to your choir.