I could go on and on about scientific facts and fallacies, the merits and disadvantages, the test's strengths and weaknesses.
But it's not necessary. After you take the test (it should be composed of hundreds of questions, not dozens), then read the description of the personality type it says you are. If it sounds like you, then the test worked really well.
I am an ENFP. If you wanted to know what I am like in real life, then go read the ENFP description. That description is the most accurate and concise description of me that I have ever found, and is much better than I could have ever written about myself.
So the test worked better than anything I've ever seen. But go read your own description. It's very easy to see if the test is "sound" for you.
A test could have a 90% "unsound" fail rate, but if you can tell 100% of the time when the test fails, then it's a 100% useful test 10% of the time.