I've done every species of CBT and "Mindfulness". All of it "works" in the sense that taking a placebo, staring at a flickering light, or praying to Thor "works". Psychology is (except for real, organic mental illness) the gentle art of taking credit for regression to the mean. If you take someone with a problem of some sort, and just wait six to eight weeks (or eight to twelve, or four to six, or six months, depending on the problem), the problem will likely have resolved itself. Psychologists and Psychiatrists merely observe that natural regression to the mean and tailor their "treatments" to essentially fill that gap. Doing something helps in most cases. Prozac, journaling, prayer, staring at the moon. Whatever. Make something up. The more inconvenient it is, the more people will think it is "working." Their "gold standard" of double-blinded, placebo-controlled "studies" is absolute garbage. It works for real, organic illnesses and things like vaccines and what have you. But for "depression" and personality disorders? It's pointless. They have no baseline and the smart ones know it. It's gotten to the point where the "treatments" that were invented as placebos are recommended as real treatments by quack and borderline quack therapists now.
So journaling will "work" in the way that prayer or meditation "works". If you have a problem of some sort, and journaling helps you to maintain focus and dedication then it will certainly "work." But it's not doing anything beyond… It's like how writing a note works when you are going to the grocery store. We took two groups of people, and told them ten items to get at the grocery store. One group wrote it down in a note, the other group had thirty seconds to memorize it. Something like that. It "works."