Hi, I'm the author of the article. Thanks for reading, and if anyone has questions, I'm happy to answer them.
I think the only downside of legalizing it will be increasing the number of users, and therefor the number of possible bad trips/stupid actions while high.
Being that we let ~88K die in the US due to alcohol, I think we can handle that a couple dozen misadventures from shrooms as well.
I'd assume that we have a lot of data with cannabis now. I'm guessing that it is a significant net win.
Drug use is so demonized that it will take a lot of time and easily messaged education to change that.
Edit: Google Scholar shows hundreds of studies, one on the first page (for me) has >700 citations.