I wish I had some words of wisdom for you, but I really don't. I've been trying to decide if I want to confront "my" band, not really my band, but the band I'm in, about problems that I see and what to do to fix them, and I'm not subtle or tactful myself, either.
Maybe start with some positives, and then ease into the "what we need to fix" part? One thing I have learned is that some musicians just can't sing harmony, and shouldn't even try. And some who CAN sing can only sing a part that comes naturally to them. In other words they can't learn a part and consistently sing it if isn't a part that they instinctively feel.
Maybe suggest a night for singers only with acoustic guitars to work on vocals? And record the process and listen back.