Because the latest official version of GEDCOM was 5.5, Family Historian does not use 5.5.1, I think the reason being that there were a few different flavours which would cause confusion. I have started using it partly because of the GEDCOM compatibility, but lots of other reasons, but an added bonus, and even if not perfect it is probably the only software to be anywhere near 100%. It saves it's database as a GEDCOM file and is amazingly fast. You can also use it as a simple GEDCOM file reader and it will try to correct errors. They are keen on the 100%, so why not try out the 30 day free trial (no plugins I'm afraid, pity as that is a major feature) and see if you can break it, I'm sure Calico Pie and the FHUG (user group) would love to hear your results.