The MBTI has some correlation to the Enneagram, and it is from that correlation that it takes most or all of its validity. But the correlation is only partial. If you look in Jung's Psychological Types, you will see that he correctly characterized eight of the nine Enneagram types (which one he missed I leave as an exercise for the reader). Missing one type, he was unable to see the whole structure with its deeper properties, such as integration/distintegration, that reflect the way the types relate to one another. Nor did he pick up on how the types develop as childhood survival strategies (which is not as simple as you're suggesting).
The MBTI starts with Jung's already incomplete system and just messes it up further.
Someone will probably ask me whether the Enneagram has been objectively validated. It has not been, and can never be, because it contains an inherent element of subjectivity. The way you know your Enneagram type is not by taking tests. Tests can provide useful hints, but if all you have is a test result -- even if it's correct! -- you do not know your type. The way you know is by studying the types until one of them clicks for you, and you see yourself in a new way: then you know.
The Enneagram does talk about our dark sides, and that's important; but it isn't only about that by any means. It can also show us positive aspects of ourselves that we have not accepted, and it's very much about how to develop those aspects further.
ETA: if you would like help figuring out your type, I would be delighted to assist. My email address is in my profile.