I am reminded of a podcast where a panel of people far more knowledgeable about the subject than I, were discussing type. One of the topics was a compare and contrast between types using Fi vs Fe in their stacking order. The two types that use Fi unanimously agreed that one way of knowing someone uses Fi is they have very strong likes and dislikes, and may not be shy about letting others know.
With that said, would someone using Fi especially as their dominant function have even stronger likes and dislikes that could be deal breakers as far as relationships go? As a Ti dominant type, I don't think I would get along necessarily well with another ISTP type if our guiding principles were in conflict. Typology is about how we cognitively process things. Our principles and values are in part based on upbringing and culture. Just a thought.
Good point, maybe too much unwillingness to compromise by both parties.
If they at least have different Enneagrams it would be somewhat workable. ISFP 4 and ISFP 9 -- could last a little while maybe