I am at a program with both accredited, non-boarded fellowships as well as accredited, boarded ones; and can shed some light. Accreditation and board-certifiable are two separate entities. Accreditation simply means the ACGME has a site visit with that program and reviews its curriculum and training based on certain standards that they deem acceptable just like they do for residency. Boarded means there's an exam administered and a certificate given by the governing body after completion of such a fellowship and passing their test. A program can be accredited without being boarded e.g. selective (surgical) pathology, but a program that's boarded automatically implies accreditation (unless it's on probation and may have temporarily lost it). Selective pathology is a term used (mainly by AMA/Freida) for ACGME accredited surg path fellowships.
Non-accredited surg path programs (or other AP subspecialties) don't necessarily have inferior training, or less merit on one's CV. However, there are unique circumstances which may apply for some where an accredited fellowship whether it's boarded or not is useful: If one registers for only one of the boards (either AP only or CP) without the intent of sitting for the other, but then changes their mind to take the second part later (which they never registered for), additional training to qualify to be eligible for the other component of the boards as required by ABP will only be counted if it comes from and ACGME accredited fellowship. The second situation is if one's board eligibility expires post-residency because that person never took the exam within the 5-7 year window. If this happens, the ABP requires remedial training. The expiration of the person's eligibility would begin after residency unless they did an ACGME accredited fellowship which would essentially buy them another year of eligibility. Hope this helps.