The only thing that matters is what comes out of the speakers.
Excuses don’t matter. The only thing that matters is what comes out of the speakers. You will not have a chance to explain to anyone why the mix, vocals, production, whatever doesn’t sound as good as it should. It doesn't matter that the drums were recorded badly, the singer's timing was bad or something was messed up with the Protools files that were delivered to you. No one cares. No excuses will matter later when they hear your work coming out of the speakers. If you have to go in and work a little harder by replacing drum sounds, editing the singer or fixing the crap that was sent to you, then you need to do it. Your reputation is on the line and your reputation is what comes out of the speakers.
If you are producing an act and the guitarist can’t play or the drummer has bad timing, it does you no good to complain and do a lame job producing them because THEY suck. No! You were hired to make a professional recording and if that means replaying the guitars yourself or getting someone else to do it then that's what you do. If it means getting another drummer to play the parts or spending the time editing the drums then you do it. Because no one who hears your work later will be able to hear you tell your story about how slack the band was. They’ll just hear the music and read the credits with your name in them.
I learned this lesson the hard way. Many years ago I was producing a band with a singer who was... a bit challenged, shall we say. She was kind of lazy too. She might come in and not be having a good night and just want to call it quits instead of moving on to other parts or seeing if she'll get warmed up. I started to get frustrated and impatient with the whole project and began rushing through the whole thing just to get it out the door. I figured that as mediocre as it was, and as lazy as the singer was, no one would ever hear it anyway.
After I got it done this singer suddenly seemed to get some fire under her butt and began hitting the local scene pretty hard. She also got some money together and opened a rehearsal facility which brought her into contact with a lot of bands in the scene. It wasn't long till word got back to me how she was bad mouthing me for the half-assed job I did. I wasn't able to go around and explain to everyone how bad of a singer she was, what a pain she was to work with, how she was lazy and how she seemed to be satisfied with the way the project sounded. Just because the client is satisfied doesn't always mean it's done! Well, word got around and it hurt me. I lost gigs and it damaged my reputation in that scene. But frankly I deserved it. When someone hires you you are obligated to do your best work. And besides, you never know who is going to hear it.
You will live and die by the quality of your work.