There is no drawback to using the couples match even for the most competitive specialties. Be sure to read all of the information on the nrmp website carefully. Basically you and your partner would rank all of the desirable pairs of matches (keeping in mind that they do not have to be at the same institution).
When you have exhasted all desirable pairs, you would have to make some decisions. If it doesn't work out with both of you matching in competitive specialties in geographic proximity, you will have to choose the least undesirable of the following:
1. both match in the competetive specialty/specialties of choice, but in different geographic areas,
2. be in the same place with one of you in a less competitive specialty
3. one of you go unmatched completely
When you have made these choices you will create the rest of your couples rank list accordingly. It is not too complicated when you refer to the examples on NRMP, but your list can get long and complex.