From what I've read, the training isn't necessarily transferable. You just get better at these sorts of brain games, which doesn't necessarily mean your working memory is increasing.
Even while reading gwern's blog that seemed pretty positive of this kind of training, there was limited evidence that you shouldn't learn just a new instrument or language or new sort of math discipline.
Why do people keep thinking that "training" can improve working memory?
I've benefited so much from this protocol that I created a mini app just for myself (https://mind-workout.pages.dev/)* as I was unable to find a suitable app for mac. [On windows there 's Brain Workshop (https://brainworkshop.sourceforge.net/)
*This is a variation of the n-back task called triple n-back, which is much harder than dual n-back.
Usually when you "learn" something you improve your understanding of the domain, you start chunking things up into patterns and structures. This reduces your mental load and lets you use your "working memory" more effectively.
I think the intuition with say, "n-back" is that there's supposed to be no structure beyond the memory task, so any increase in performance _must_ be an improvement in some sort of generalised "working memory".
As I understand it people have shown that there is "transferance" between these various types of working-memory based brain games (i.e, getting good at one can improve your performance on others that you haven't done before). But no one has shown that getting good at (say) dual n-back produces a strong improvement in "real tasks" that aren't just memory games.
It only takes 40 mins a day for 8 weeks to test it out. Much less time than the commitment to learn a new language.
Having tried it, I wouldn't be surprised if the mixed results were due to improper adherence and misunderstanding of how n back works by some study participants. In other words, I think it's possible that results would be less mixed for someone who is already starting from a point of solid intelligence and who is driven enough to put in the hard,focused work to get to higher n back levels.