Please accept our apologies for the inconvenience caused. Indeed, the YAF integration has been on our mind for several release sin a row, actually what made us give it up was not the fact that custom membership providers do not work (I can assure you they do, as we've had several clients who've been communicating with us in the process of successfully integrating custom membership providers in their Sitefinity projects).
The problem we've encountered was that there was no way to integrate a common membership and role provider between YAF and Sitefinity (in other words YAF using the default Sitefinity provider, so the users can be shared). Due to the above, the YAF controls did not work properly on a Sitefinity 4.x page. Once again, please excuse us for the inconvenience, from what I can say the CT.Forums module has been a successful alternative so far, and we've had satisfied clients using it.
I'll make sure to include the matter in one of our internal discussions, when planning the upcoming releases, and will let you know if there's been some further progress.
All the best,
the Telerik team
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