I've wheeled Land Cruiser SF axles for years and years and years... and have yet to see one actually fail. I parked my BJ42 at almost 500,000kms and it never had an axle fail running tires ranging from stock to 36" TSL SXs with ARBs at both ends (sprung over, dual winches, dual lockers, Saginaw PS etc). I ran my FJ45 (FF rear, Lock Right, TBI 350, SM420, Saginaw PS, SOA on 36s [had run both 35s and 38s intermittently) pretty hard and wheeled it for several years before selling it - and never a problem with the FF rear axle.
It is very rare for the SF axles to break, they are HUGE at the wheel end, and have the standard 1.31" 30 spline portion in the diff at the other end.
I know of not a single person in any of the groups I've wheeled with to ever break a Land Cruiser SF axle. I'm sure it happens, but I've just never seen it or heard of anyone in my circle of Land Cruiser owning friends have it happen to them. I asked around in the shop and guess what - no breakages experienced or known about first hand.
FF axles typically break in one of two places: at or near the spline - in which case it can be almost impossible to get them out depending on how it breaks off (there's no rear cover to remove to access the broken bits), or at the flanged outer end due to (usually) running the wheel bearings too loose causing very small amounts of flex to work the material until it fails. Both instances are very, very rare.
The other failure is for the FF axle to strip off all the studs at the flange (x6) and the dowels (x2) due to using non-factory gaskets which were (most commonly) replaced with silicone. Tip: use genuine gaskets on a clean, dry surface with (fresh) studs torqued to spec.
Yes, you can run a FF axle without an axle shaft in place, but it's pretty uncommon to even break an axle if you even half way keep up with your maintenance.
~John