The OP asked about the SM7, not the SM57.
All mics seem to work better with some voices than with others. IME, the SM7 seems to be at least good, if not excellent, with more voices more of the time. As a dynamic mic, it is more forgiving for those of us who have less than perfect recording environments. The lower sensitivity, compared to a condenser, means that you are less likely to be picking up extraneous noises when recording.
I find it a well balanced mic that sits well in a mix without a lot of EQ. And you do need to try it in a mix, rather than just soloing it, to see how it sounds. It does not have a particularly hyped high end like many of the modern condensers, although it does have a presence boost switch if you want it. I always leave mine flat.
I started off with one SM7 that I bought used. It kept getting selected for vocals instead of my other more expensive mics (GT MD1a, GT MD3, AT-4050, Beyer MC834, Neumann Gefell M582h, Audix VX-10, Sennheiser MD-441, Beyer M-500, AEA R84). It also got used for guitars, cabs and toms. In fact it got used so much that I now own three of them.
Now, I certainly don't have every mic out there (I have some friends currently using one of my SM7s and my vintage U87 - we'll see what they think - last I heard they were still going with the SM7 for vocals), and this is not to say that there isn't some other mic that might be better on a particular voice, but it has seemed, in most cases, that the SM7 has been the talent's choice. I would note that we like the AEA R84 for BG vocals where we can do them as an ensemble and back off of the mic.