Any experience about radially-laced hub flanges actually breaking?
While it is possible for a hub flange to fail due to radial lacing, if the wheel is built properly, to proper tension, it doesn't happen, generally.

The reason it happens is because the amount of material between the spoke head and the edge of the flange is least when the spoke is pulling directly to the edge.  Under normal usage, the flange has more than enough strength, even when laced radially.  

However, if the wheel is laced with too much tension, or if the wheel is placed under abnormal stress, like when used for braking on a fixed gear, then the hub flange is more likely to fail when radially laced, than when laced with a proper crossing pattern.  

It can still fail, even when crossed, but it is far more likely when laced radially, due to the smaller amount of material, and more direct tension on the flange.

I have used radially laced wheels, mainly for BMX racing as a kid,  I have had only one failure of a hub flange, though, and that was on a Bontrager Race XXX Lite rear wheel in 2003. That wheel uses a radially laced, paired spoke pattern, which I do believe contributed to the failure.  They are also excessively high tension wheels, due to the low spoke count, and I think it took both the radial pattern, and the excessively high tension to cause the flange to break.

I hope that helps.