Going to actually try to answer most of your questions. I dated three INFJ's before and one of them lasted a few months. Answers are in bold.
- How compatible do you think these two types are in general?
I think if an ENTJ has a well developed Fe than it could work great.
- Why are they attracted to each other?
Generally the INFJ admires the ENTJ"s "thick skin" and bluntness. INFJ's are famously known to detect lies very easily, and hate deceptive behaviors. This is where the INFJ will initially be attracted to the ENTJ, their bluntness and honestly (even if its selective honesty) will turn on the INFJ. All INFJ"s I dated where extremely attracted to my bluntness. But the ENTJ's straightforwardness can be a double edge sword to the Feeler type, who might get offended or deeply hurt.
- How to they compliment each other?
The I / E difference is an obvious compliment but the F vs T can pose some problems if the ENTJ doesn't have a well developed Fe, or the INFJ is to sensitive.
- How well do they understand each other and why?
INFJ's need a strong emotional connection with their partners in order to fully commit, or progressive in a physical relationship. They are not ones to sleep around. The ENTJ might not understand why the INFJ is delaying in investing in a relationship with him, and might take it personally. Also, the I types have a strong need to recharge or be alone at times. The Extroverted ENTJ might not understand why the INFT constantly has the need to turtle occasionally. The INFJ's "turtling" coupled with the slow progression in a physical relationship might be taken as rejection to the ENTJ. INFJ's have a great tendency to hold things in. This further stunts the growth of both partners.
- What are they like together raising children?
Can't speak from experience, but from what I read ENTJ's tend to be very militant with their children. INFJ's will be more empathetic. Looks like a perfect set up for "bad cop good cop" parenting style, lol.
When it’s not working – What are the challenges when two people of this type are in a relationship?
INFJ's have a problem opening up, or expressing themselves openly. This poses a big problem for the blunt, expressive, no games ENTJ. Another challenge is the ENTJ"s bluntness can be emotionally taxing for the "Feely" INFJ.