Aries and Capricorn are both very strong signs. They are in astrology considered cardinal signs. Cardinality in astrology means that they are assertive, forward, take control types, the kind of people who don't take a back seat. So they each have that quality of being strong in their own right.
Also there is a good sexuality between the two. Aries tends to be very passionate and love making love. Capricorn has deep passions and can be extremely affectionate.
The difficulty, however, is that they are so strong in their own right that they are going to run into each other's control. Aries wants to be the leader. Capricorn won't take orders. Capricorn is the one who wants structure, who wants a life that counts for something. Capricorn sets the rules. Capricorn can be very domineering. Aries doesn't want this. So they run into these issues of who is taking charge.
In general in astrology, the signs that are that strong have difficulty making it long term. However, what I have noticed as an astrologer is very often the strongest signs that have these issues are the ones that can make the strongest marriages. It depends on how willing each is to commit to that marriage or that relationship and work with it.