The XCD 28P is not a bad lens. Of course, it's always possible to get a bad copy of a lens. For instance, if a retainer for a lens element becomes loose after it's passed inspection and been shipped; you could find an element is out of alignment when you start using it. If new, it could be exchanged or sent it in for a warranty adjustment to bring it back into spec.
From the images I've seen and the comparable MTF charts, the XCD 28P compares favorably to the HCD 28 mm f/4, Rodenstock HR Digaron-S 28 mm f/4.5, and Leica Elmarit-S 30 mm f/2.8 ASPH — all of which are larger, heavier, and more expensive. The XCD 30 mm is a better performer, it's exceptional; but it's also larger, heavier, and more expensive which are its trade-offs.
The 28P appears to me to be an excellent choice for its overall combination of size, weight, cost, and performance