People say they're broken cause they're THEORETICALLY broken. Weaknesses? What weaknesses? Moonlords cover everything. If there's a weakness other classes have, they're already theoretically not as great as Moonlords, unless their strengths outweigh their weaknesses enough.
If you just completely rely on results and nothing else, your view will also be biased and skewed.
As for why they aren't on the top 10, because in a tournament setting, despite people wanting to win, they will not always choose the best character. Not a surefire reason, but a possibility. The Evo2011 BlazBlue results had only 1 player in the top 8 using the "broken" character, and they didn't even win. Some people are just incredibly, incredibly, good, able to overtake the advantage their opponents have and come out on top with a character not as good.
This is rated PvP too, so it would be helpful if we could find the results of tournaments instead.