A properly designed fractional rig will have mast bracing to suit such as diamond jumper stays. Hence it will be just as strong as masthead, all other things being equal. However, some fractional rigs are intended to create mast bend, and the mast might be more flexible (read weaker) by design.

Hi Browny,
Horses for courses isn't it? If you have a design with a thin whippy mast with lots of bend, it will have a different max loading to a solid inflexible mast section. So, maybe less of an argument about fractional v masthead vs mast structure?
I run a cf fractional rig without runners or backstays, and we've had full sails up in 40+ knots (not by choice). We'll run kites in 30 kn, again no problem.
Look at a Mumm 36 rig. Perfectly fine until you forget the runners in a decent wind, then patooey! (is that a nautical term?)

Bendy masts with inline spreaders are a thing of the past and racing only like Mumm's .
I'd always prefer 7/8th rig with swept spreaders for cruising as the stay tension induces mast bend and stops inverting.
At 7/8th you don't need massive spreader angle to keep forestay tension as backstay will still help that.
I'd also have double spreader with inner forestay with staysail and runners at the top spreader.
Although I don't really like the mass produced mast and components, modern Hanse's have a really nice cruising rig plan

I agree Chris, after using all the adjustments I can do with the fractional rig and seeing the results i dont know how i sailed with a masthead rig