1993-2001 all had the same engine (KA24DE), so I have no idea why the previous years would be rated higher on engine quality than the '01. But this thread could be some help:
Why people think Altima is a reliable car? - SUPERNISSANS.NET Nissan Owners Community
Used cars vary man. Basic wear and tear and how well the car was taken care of all affect the quality of the car, and what kind of condition the car is in. Take it to a mechanic and get a overview of the car you are interested in buying to get a idea of the overall condition of the car. Thats one way you can definitely find out what kind of condition the '01 you are interested in is in.
