How sturdy are rigid carbon forks?

No, the entire industry is based on shullbit.
It would be nice if one of the magazines did some impartial scientific testing of components now and then.
Even something as simple as "We clamped 10 handlebars in our test rig and hung a 50kg weight off the end of each. Handlebar A deflected 5mm, handlebar B deflected 3mm etc." would help.
Why should they bother though, when they can waffle on about "super stiff construction" and "race proven technology" while taking handlebar A manufacturer's money for a full page advert and people still buy the magazines.
No chance of them doing any long term cyclic load testing to failure on forks.

The best you can hope for is anecdotal evidence from other users.
Two things I've learned from this thread are that I could have bought my forks cheaper with a different name on them and that the only two carbon MTB forks that I've heard of failing both failed at a metal component.

nuke proof, wide WHITE industries, on-one, someone else and exotic are all made in the same factory i believe. If you want proof just compare the products in detail. The dropouts/crown are the obvious places.

the on-one forks are significantly different

IME, the only carbon failure I have encountered* was catastrophic, and it wasn't a pleasant experience. Having said that there is carbon a plenty on one of my bikes, but none on the two I would choose to ride seriously off road, all components have been switched back to alu on those.

*SDG seatpost snapped in two about 30mm above the seat tube intersection on a hard landing, causing me to fall down a shallow bank whilst clipped in, intern leading to the (sharp) remains of the seatpost contacting my leg, not nice.

It would be nice if one of the magazines did some impartial scientific testing of components now and then…
Why should they bother though, when they can waffle on about "super stiff construction" and "race proven technology"

I've got to say, I'd rather buy a race proven component than a lab proven one. If component X can hurtle down the Nevis Range in under 5 minutes without breaking then I ain't going to break it. Granted lightweight XC race stuff may be a different kettle of fish, but I suspect the majority want to hear/read about real life riding experiences of bikes and components and not lab facts and figures.

…but when my White Brothers carbon fork snaps I may change my opinion 😆

2unfit2ride – Member

SDG seatpost snapped in two about 30mm above the seat tube intersection on a hard landing, causing me to fall down a shallow bank whilst clipped in, intern leading to the (sharp) remains of the seatpost contacting my leg, not nice.

I had a seatpost snap about 20mm below the clamp. Saddle bit fell off as my weight hit it and the upstanding shards missed my gonads by a few centimetres.

The seatpost was made of aluminium alloy.

It would be nice if one of the magazines did some impartial scientific testing of components now and then.

No thanks, I don't see any of the cycling mags being in a position to produce anything scientific. You'd end up with something like the disc brake test on bikeradar recently, at best irrelevant if not actively wrong, with some graphs and numbers to make it look "scientific".