It's not controversial that there are funds with high returns over long periods. The problem is that there are also funds with high returns that suddenly nosedive, and new funds from people with high returns in the past that doesn't do well at all.
In other words: it's easy to pick the lottery numbers with hindsight.
> there are US based asset managers with private funds that generate 20-30-40% per annum, for over 20 years.
That seems extremely dubious. Do you have any sources?