Were it the case that most secret societies work for what's best for all people of good will, I'd be less inclined to oppose them. But history suggests otherwise. In the end, it often happens that the secret societies work for what's best for their own members and their own interests, and if that happens to coincide with a wider good, okay, but if not, too bad for the wider good.
But perhaps I'm too cynical...
There's the problem. Opus Dei is not a secret society. It would never have been confirmed by the Holy See if it were. It is hated by liberal governments in Europe and dissident Catholics in America because it is a strictly orthodox organization in the Church which is having an effect in it's apostolate.
The "secretive sect", "controversial", etc. ad naseum, labels are applied to Opus Dei as a scare tactic. Reminiscent of claims against the Jesuits in early 20th Century America.