Part 1: Effective vs. Ineffective Methods
Note: what I meant to say at the end is that it's not contradictory to say that it MAY be the case that, while there are universal principals of language acquisition that are true for everyone, certain specific methods may be more cut out for some people than for others.
Part 2: Avoiding “Universally Ineffective” Methods
I forgot to clearly state that, when “testing” people who are giving out language learning advice, the thing to really look for is whether said person has achieved what you are specifically hoping to accomplish. In other words, if you want to accomplish “x”, only take advice from people who have successfully accomplished “x”. A corollary of this is that, even if someone hasn’t reached the highest possible levels of foreign language mastery, as long as they have reached a level you would be happy with, their advice may be worth listening to.Something else I forgot to mention is that another reason you can’t trust people’s self-reports about their own language ability is the Dunning–Kruger effect. The Dunning–Kruger effect is a cognitive bias in which, for any given skill, people at relatively low levels tend to overestimate their ability, due to not being competent enough to recognize their own incompetence. In language learning, this often manifests as intermediate language learners believing they have reached a very high level in a language, due to comparing themselves to other language learners. When these intermediate learners begin to reach a more advanced level, they usually transition to comparing their ability to that of native speakers and realize that their level is actually quite low. For more on the Dunning–Kruger effect, I highly recommend this short video.
And lastly, I am no exception to these principles! Don’t just blindly assume I know what I’m talking about! Go “test” me as well as other popular figures in the language learning community. I want you to have confidence that the methods I talk about on this site are proven to work. You can find videos of me speaking Japanese here and here, and a video of Khatzumoto (creator of the AJATT method, which inspired MIA) speaking Japanese here.
I also encourage you to explore other methods in addition to MIA, especially if you ever run into an issue that MIA doesn’t seem to provide a specific solution for. Even when using the best methods, it’s important to take personal responsibility for your learning and results.