Ewan, within the dates given they are intended to be complete. If you look at the list of IOMFHS burial transcripts here: www.isleofman.com/heritage/genealogy/fhs/publications/burial.aspx
you can see what Brian used initially to create his index. If any were (inadvertently) missed from the FHS transcripts, they will also be missing from the index, unless someone sent a missing entry to Brian. There are also a few errors and typos which Brian would correct if they were sent to him. Some parishes are missing because they were never transcribed by the FHS (e.g. Ramsey).
The missing Braddan volume was found when Brian was very ill, so those may be missing - see these on the FHS website.

Sue