I take back what I said earlier. I think both of these types probably need time to develop their functions and would do better with each other in their thirties, but they have very complementary strengths.
Entps have a high tolerance for and interest in complex problems and if it's important to them, have the skills to allow the infj to feel understood and get them to verbalize their thoughts and feelings more easily as well as draw them out of themselves. They also are good at getting them to try new things that they wouldn't on their own and infjs appreciate someone who is able to do that.
Entps can quickly read people and get a grasp of what it is that motivates them. They also not just tolerate, but enjoy solving problems in the relationship and can be very creative with finding ways to develop new layers of meaning in the relationship. I think that with infjs, there is less pressure to always be "on", than there is around other people, partly because the entp has more to engage with, and because the infj wants to get to know more sides to them and shows an interest in that part of them rather than just the caricature version of them.
Infjs I think serve as a bit of a stabilizing force. Their Ni makes a nice counterpoint to the entp's ne and they see things in a novel way, which entps appreciate. They often can reach similar outcomes but get their in completely different ways, which both find interesting to investigate. I think infjs serve as a good sounding board for entps, which they often don't have, and allow for them to explore ideas and feelings in a way that rarely happens with others. I appreciate both cleverness and a bit of bluntness (not possessing either quality in the way they do), and I think entps like having someone recognize their strengths beyond solely their value as entertainers.
Both types are very reflective, and are in some ways self aware, but in other ways are not terribly aware of how they feel or operate emotionally. They are both better looking at others than themselves and so I think have something to offer one another in that regard.
I think over time, infjs could find the ne lack of stability and constant flux disconcerting and sometimes grow to distrust that what is true or promised in the moment will be true later on. Particularly with a family together, I could see how that would be difficult. I also think that infj's preference for stability and convergence in thinking could be rather stifling or alarming to an entp. I also wonder if their need to conserve emotional energy through prevention or avoidance of certain situations would be hard for an entp, who is can easily adjust on the fly.
I can see how an entp with poor character or who is dishonest could potentially wreak havoc in an infj, and an infj with avoidant tendencies and poor communication would also be disastrous. If both parties were both healthy and mature though, it seems that there are some good possibilities.
Overall, I'm a pretty big fan of this pairing under the right circumstances and think that if both parties are well developed, it has a lot of potential. A lot of problems that others might struggle with could be overcome.