I have questionable Synastry with the girl I've been seeing on/off for 2 years, but the Composite is a powerhouse with lots of Sun/Pluto/Scorpio/5th house. Our Synastry has a lot of squares, so there are often a lot of misunderstandings in communication and emotion, but we always work through it and "transform," even if it's just a friendship now. But it's always had a hard time getting off the ground as a relationship, likely because of those difficult Synastry aspects.
Meanwhile I had insane Synastry with my ex before (conjunctions everywhere), and to say there was a spark would be an understatement. But the Composite was pretty bland and detached, and it fell apart despite both of us thinking it was special initially.