Oomis, wow, that could have been me speaking, but you said it better than I would have.
I am an FFB married to a BT and am blessed with the most amazing husband and in-laws, but many issues arise, especially as our children grow, that require tact, understanding, and flexibility. If an FFB does not possess these traits, s/he could compromise the relationship with in-laws, and possibly with spouse.
Many FFBs married to BTs have strained relationships with their in-laws; then again, so do many FFBs who are married to FFBs. The traits of tact, understanding, and flexibility are required in any relationship, only more so when the religious and the secular blend into a family.