Of course it always depends on the persons moon sign and this and that etc, however as far as ive seen its a tough one, the capricorn i knew would kinda shut himself away from his cancer woman because he couldnt handle her emotions, which he appeared to be a jerk later in their love life to her. But as always if both couples communicate and are willing they can make it work. But BOTH have to be willing, good luck:)
Honestly speaking, they supposed to have the best compatibility despite they are opposite on the horoscope. Its a bit shaky. They will have fluctuations in their relationship but they will go in for the long run if they see past their flaws. In any case thats what it takes for any relationship. So wish you the best.