The problem is that it seems that the USCIS is pretty good at processing applications that stay on the "fast track". If your N-400 goes in, your fingerprints get done on time, and your name check clears, you can get and interview and an oath in a small number of months.
But...
They aren't very good about processing applications that get out of the "everything is working well" pile and into one of the other piles. It's real easy to suddenly end up looking at a 3 or 6 month wait for them to get around to rescheduling something.
As an example, my file didn't arrive at the Dallas district office on time for my interview. So, a little less than a month before the interview they sent me a descheduling notice. My file did finally make it to the DO about right around when my interview would have happened.
In the end, my interview did happen, but 4.5 months later. If I hadn't got my senator involved, I don't know how long it would have taken.
The moral of the story... Don't give them an excuse to take you of the fast track.