bro, i won't say that nd filter is good for sunset or sunrise. after all, i use it a lot during daytime..
it is good when you want to use it... for the effects that nd filter bring, i.e. long exposures to smooth out water, to darken the scene when you want to shoot wide open and using the lowest iso with the fastest shutter speed gives you an overexposed photo... etc.
yes, it will degrade the image somewhat. especially if dirty. but if clean, should not be visible.
as mentioned, when you have a concentrated light source directly shining on the filter/lens combination, the filter might cause flaring/ghosting.
i don't use any uv filters... as far as i'm concerned, i don't see how they help in "protecting your lens"... if you are shoving your lenses into things that can scratch the front element, you need to learn how to protect your equipment. putting a uv filter in front gives one a false sense of security, if you ask me. after all, there have been recorded incidents where the uv filter broke and scratched the front element... so it can be as much of a bane as it is touted to be a boon.