FTM drops many 2,3,4 and 5 level tags when importing into it's database.
Any Level 1 tags starting with "_" are subject to being dropped by other genealogy software programs when they import them if they do not understand what they mean.
Sometimes these drops are noted in error reports sometimes they are silently dropped.
As previously noted, many people just stay with FTM and are very very happy, and don't care about GEDCOM. I don't want to alarm you any more than I have, about this issue, because for most users IT IS NOT AN ISSUE.