Keep out areas can be a problem when adapting a CAD component land pattern, but that's not the only potential problem. Sometimes the part may be close, but the footprint is different enough to cause problems, as in the picture on the right.
You can also run into issues that don't necessarily cause PCB assembly problems, but can be expensive none the less.
Say you are designing with a small microcontroller and the schematic symbol and land pattern don't exist for the one you're using, but something close does. Even though the two parts may look like pin for pin replacements, they may have a few differences.
The PIC family has a number of examples of this. For example, the PIC18F2321 and the PIC18F2455 have enough similarities that they look like pin for pin replacements. However, upon closer inspection, you'll find that RC3 exists on the 18F2321, but doesn't on the 18F2455. SCK/SCL and SDI/SDA are in differnt places on the two processors. You could end up with a bunch of jumpers and a PCB re-spin if you just used one land pattern for the other. It pays to check for those little details.
Turn left at the big tree, and go until you see the creek.