NGC 4565 is spectacular, but when all is said and done, it's an edge-on spiral, and edge-on spirals tend to have pretty low surface brightness.
And Roger Corbett corrected me:
As far as I've observed over the years -- and as confirmed by observing reports and expert commentary here and elsewhere -- edge-on spirals have GREATER surface brightness than face-on ones do.
He is quite correct, of course. If you ignore the presence of dust, then the total amount of light that you can see from any given galaxy would be unchanged if you could somehow reach out and twist it to any angle you wanted. Since the total light is constant, and the apparent area is bigger when a galaxy is face-on than when its edge-on, that means that edge-on galaxies generally have higher surface brightness than face-ons.
However, galaxies do have dust, and when viewing NGC 4565 -- or our own Milky Way, for that matter -- the dust happens to block out most of the galaxy's core, and therefore a large fraction of the total light.
It's the absence of a bright core that makes both NGC 4565 and M33 hard to spot, especially in light-polluted skies. In the case of NGC 4565 and some other edge-ons, dust is the culprit. In the case of M33, it's because the galaxy is a bit of a freak, with an abnormally small and faint core.