How well it works, including where it does not
Here is the honest picture, including the number that most marriage material leaves out.
What the research says
Rathgeber M, Burkner PC, Schiller EM, Holling H (2018). The efficacy of emotionally focused couples therapy and behavioral couples therapy: a meta-analysis. Journal of Marital and Family Therapy, 45(3), 447 to 463 doi
33 randomised trials, 2,730 people. Behavioural couples therapy did better than no treatment at the end of therapy, Hedges g 0.53, less so at six months, 0.35, and by twelve months the difference was 0.06, which is close to nothing. Emotionally focused therapy did better at the end, 0.73, and held at six months, 0.66, with no twelve month data available at all. The authors themselves list serious limits: some full texts could not be obtained, only four languages were included, publication bias was detected with around 15 studies estimated missing, long term data are insufficient, samples were mostly white, heterosexual and North American, and most couples were moderately rather than severely distressed.
Bograd M, Mederos F (1999). Battering and couples therapy: universal screening and selection of treatment modality. Journal of Marital and Family Therapy, 25(3), 291 to 312 doi
This is the paper behind the gate at the front of this course. It sets out why every couple should be screened for violence before any joint work, because it does not reliably come up when a couple are asked together at a first appointment, and why joint work is not safe where one partner is controlling the other.