I have the same issue with poor quality, which I never experienced before on FIOS when I was recording on Windows Media Center with a CableCard.
I believe the issue is simply with overcompression of the video signal (at the broadcast source or by recompressing on the DVR, I'm not sure). Running a fiber network removes most bandwidth issues, but to maximize DVR storage, my guess is that they overcompress some or all channels...maybe they reserve higher quality for premium channels like Sirius/XM does, I'm not sure. But for most programming there's significant ghosting and banding/posterizing that is more obvious in darker areas of the picture...it's completely unacceptable and I don't know how they get away with it.
DVRs have been around for almost 20 years, and from the beginning they've given users the ability to choose between capacity and quality...Verizon, it's bad enough that you don't even have 4K delivery on the near horizon, the least you can do is let your customers choose their compression level!