Disastrous: Causing great damage. Fortunately (heh, see what I did there?), I have never experienced a disaster. Neither of the natural kind, nor the personal kind. And that leads me to the word's origin: From "disaster" which means "ill-starred" or to be ill-fated because of the stars.
Modern astrology is more psychological and less about fate, more about choice and less about destiny. If I really want to scare myself all I have to do is read an old book and my own birth chart will horrify me. I should be in prison or a psychiatric hospital, or perhaps married to a violent man whom I stay with because he's stinking rich.
Here's where I like the modern stuff better: Understanding that a birth chart is exactly that: A chart, a map, not a reality. You can experience your map in many different ways, and how you do will be colored by the people in your life and your circumstances. We all share a birthday with many people and none of us are having the exact same life. I spent part of my childhood in Norway and that affected how I would travel my map.
It's not just the position of the planets at birth, but also how they move and interact with the birth chart over time. The hands on the clock move and at certain times things are more likely to happen. A bad transit can be disastrous—or not. The difference between the "lucky" and the "ill-starred" probably belongs to chaos theory and not astrology, anyway. Something said or not said, an accident, a temporary decision that became permanent, the butterfly wings that flap during childhood and take us somewhere we never expected.
One disastrous event in my childhood was my parents' divorce, which I was not handling well leading me to self-destructive behavior, and some situations that for an adult would have meant prison. I did not stay on that self-destructive path. Instead, I grew up in a safe place, with loving grandparents, exposed to transcendentalism and affirmations.
One suggested antonym for "disastrous" is "heaven-sent". How about that.