How effective are reforestation projects?

It very much depends on how they were approached and managed, and of course “industrial-scale” reforestation has really only been considered for the last few decades, and implemented for even less so the jury is still out. On an individual basis though they can be wildly successful, for example

https://www.npr.org/sections/parallels/2017/12/26/572421590/hed-take-his-own-life-before-killing-a-tree-meet-india-s-forest-man

And

https://www.bbc.com/news/av/stories-52122285

But there are numerous examples of monoculture reforestation and attempts that just failed altogether as well.