Well I'm not a producer, but I play in a band and I can at least tell you how I see this from a musician point of view.
I'd start by saying, as was mentioned before, that it is extremely difficult to state that "something sucks", since by definition, it can't really be objective. Saying that something sucks is totally an opinion.
The only way it could be an objective statement is if the song IS UTTER SHIT by all existing standards. Like, it's the sound of someone defecating in the mic for 4 minutes with background noise of washing machines... I don't know. To say that "it is a shitty song" would be quite close to being objective.
Apart from that, you can probably find "what sucks" in your opinion and just say it, with your outmost diplomatic tone. Either you find the lyrics are dumb, or the singer is bad, or whatever. Saying, in a more diplomatic manner "The singer is bad", is completely different and more useful than just saying "The song sucks".
So I don't know if you get what I mean, but I just wanted to point the difference (difference between saying it sucks at large, versus saying what sucks precisely)... Music is so subjective, thousands of band make music that in my opinion, is shit. Yet they sell albums, go figure...
The day you'll make millions producing albums, then go ahead and tell them whatever you want!
(I know I haven't heard the band in question, therefore take what I just said with a grain of salt. But that is how I see things.)