The SM7b is a very low output moving coil microphone. Having eventually found the specs, which are hidden on the Shure website well away from the end user's prying eyes, they're still not entirely clear.
Ok. Since this is ASR, we can do some "back of envelope" calculations...
SM7B sensitivity is quoted at -59dBV @ 1Pa (i.e. -59dB referenced to 1V for an input sound level of 94dB SPL).
Although equivalent input noise is not explicitly stated, they do mention a strange entity: "hum pickup at 60HZ" = 11dB (equivalent spl).
This is actually meaningless as it is quoted, and I suspect that it should have read "equivalent input noise=11dB SPL", but has been corrupted for some reason. Assuming this is the case, then this is a pretty good figure, and much as one would expect from a mic which is simply a coil of wire attached to a diaphragm.
However, due to the mic's low sensitivity, the output level of the noise (hiss) would then be -59-94+11 = -142dBV (=-140dBu).
To realize this low noise level in practice, requires an extremely low noise mic preamp. The best pro-spec mic preamps have an equivalent noise of around - 130dBu. If you want lower than that you need to use a step-up transformer. So the input noise of the preamp is likely to dominate.
Your Motu M4 is specified to have input noise = -129dBu. Given the sensitivity of the SM7B microphone, this will result in audible noise (hiss) at a level equivalent to 22dB SPL. This is perfectly ok for close-miked vocals, but you might notice a slight hiss if used at a distance to record acoustic music. So not the best choice for a lute recital!
I guess that the anecdotal reports of high levels of noise are related more to poor quality preamps than the mic itself.