The honest limitation: if a door built in this window has two or more panels bowed, cracked, or badly rusted, panel replacement stops being the economical choice. At that point you're paying for parts and labor on a door that's already near the end of what a 1970s-era section can be expected to give you, and a full door replacement often costs less over the next few years than chasing panels one at a time.
The other place it goes sideways is matching hardware. Older doors in this area sometimes have hinges or rollers that were swapped at some point for parts that don't quite match factory spec. When that's the case, a new panel can look right sitting in the opening but bind on close because the hinge geometry is slightly off. We check this before ordering, not after installing.
We don't guess at a match sight unseen over the phone. If you're in Oakland Park and a panel's damaged, we'll come look at the actual door, the actual hinge spacing, and tell you straight whether a swap makes sense or whether replacement is the better money.