If one person has trauma related problems it is recommended for that person to be working through them individually in therapy. As far as couples therapy, you are correct that one person may hold more challenging behaviors that affect the relationship however it doesn’t change that in order for couples therapy to work, both parties have to be willing to put in work and not blame each other. it is very challenging to have someone accountable and hold responsibility while not coming from a place of blame. This might include one person having to accommodate or bend more. In therapy it may be found that the person having to accommodate more may feel like it’s too much.