Keep in mind that astrology is just one part of the equation. You have to also consider level of awareness and mood. You could be with your soul mate and don't even notice it because s/he is always in a bad mood and not really herself/himself and so the relationship won't be satisfying for anyone involved. Or you could be with your soul mate but you would have very different levels of understandings and awareness. That would be harmonious but in the long run not very satisfying either. Relationships are a very fascinating but rather complex matter.
About opposites and so called challenging aspects, personally I think aspects don't matter that much, especially when you are on your own and with yourself because even if you have contradictory habits or character traits, you probably won't even notice because that's just the way you are wired. But others may notice it because they have a certain image and expectations of you which you then sometimes may contradict or not fulfill. And so thru them (or social norms) you may realize those 'challenging' aspects. So you will probably notice them more indirectly than directly.
But you are right, of course, opposites do attract, I see it here on the forum all the time when I look at synastry charts. Contrast is essential for expansion. And expansion makes you feel alive and joyous. That's the nature of dualistic feeling states. You can only feel joy if you also know sorrow. If you would only feel joy for a while, then you would get used to it and it would feel normal and become your habitual state. That's called normalization. Same goes for sorrow. So if you really want to be in bliss again and again, then you have to expand because what you call bliss today will be the new normal tomorrow and old news the day after tomorrow, i.e. the freshness is gone. If we apply that to relationships, then we could say that 100% harmonious relationships are not even wanted and even counterproductive if your goal is to live in joy. The trick seems to be to get the right amount of contrast, because if there's more contrast than you can handle, then your will feel overwhelmed and if there's way less contrast then what you are used to, then you will feel bored. Both overwhelment and boredom are negative feeling states. And what I've noticed is that the more self-aware one is, the better one can handle contrast. Which means the more awareness there is, the less those astrological factors actually matter. After all, your day to day mood does not depend on astrological factors.
We cannot ascertain from a chart if someone is going to be mostly happy or not. That depends entirely on what one is doing with his/her focus and conscious thought. And in the end, isn't someone who was usually in a happy mood considered to have lived a happy life and someone who was in a bad mood most of the time considered to have lived a sad life? So in that sense, there are no good or bad charts. However there may be some charts that can make it easier to go with the current trends of society than others (I noticed that in this day and age with such a strong focus on marketability those with an emphasis on water signs tend to have more difficulties finding their way then those with more emphasis on the other elements).
Interesting conversation about a fascinating topic!