Parts for the rebuilder have always been an issue. The piano industry is relatively small, and the rebuilding end of it is relatively very small. Since even generations of Steinway whippens (one of the most unchanged entities in the business) display differences from retooling, there is no way a rebuilder can order and receive exact duplications of what is being replaced. Deviations of 1/32 of an inch, particularly in concert with other deviations, have consequences that demand attention to produce the correct end result.
In the action, if the end result has the expected blow distance, dip, and aftertouch, if it displays the expected down-weight, up-weight, and inertia, if it repeats properly, has enough expected resistance, and has power, evenness, and musicality, then the parts work. But they do not work this way out of the box. Only careful preparation and accommodation solve the maze of inter-considerations.
Deciding on the best parts for a particular piano is often a debate involving a complicated grid of pros and cons, deciding which cons we can best work around and which pros are most essential. Parts making has become more competitive, however, and the makers are responding to feedback. The choice today is the best it has ever been.
Restoring original parts, on the other hand, has a much longer list of difficulties, compromises, and inconsistencies. Where once it seemed a hard choice, for the most part, new parts offer the better long-term solution today.