Regarding the axle spline/dia problems - note that when ordering an 818 from FFR they required you specify the donor you were going to use. Because Mark shows axles with FFR part numbers, and from my post of FFR part numbers it is clear that FFR had at least 3 different axles they might supply, depending on the expected donor. And we all know that the aftermarket also provided rebuilt axle/hubs with their own format of splne setups.

Add all this up and we are left with a lot of mix or match possibilities, no fault of any one source. Subaru definitely had multiple versions of axles over time - I had at least three versions end up in my scraps pile here during my project - and some were constructed way beefier than others.

During my waiting period of several months in 2016 for my kit to arrive, I read every build log posted here. There were several run-ins between builders and their axle/hub combinations that did not fit. Guys spent time getting and trying different parts from NAPA, RockAuto, the scrappers and FFR. I was surprised it took me only one try with FFR to get a different axle and it fit my 2008-2009 Legacy hub set which was JDM parts imported off a Japan sourced donor!

...another FWIW ...
You can also find WEB blab not related to FFR/818 at all, where people muse and guess about the axles and drive train parts in the OEM engineered drive train maybe being less than as strong as WE might want in a rear only drive setup. Translating those opinions suggests to me that with all wheel drive, the donor systems would have shared the torque amongst the axles so no single axle had as much stress as we might put on them in the 1/2 as many parts = twice the torque (sort of) in the 818. But then again, the newer Subarus (not sure where this happens at in model years) have more sophisticated power train management - which suggests to me at least - for such newer systems, a power train computer just might put a lot of torque out on one, two or three rather than all four wheels for some road condition... i.e. newer donors might have beefier parts.

Now in my case, running a '98 manual transaxle on the H6 3.0 motor, which was almost never sold world wide with a manual, (stories exist some Aussies got manuals if I have it right), I consider my car at high risk of snapping an FFR axle the first time I slip my foot sideways off that clutch pedal. I suspect getting axles from FFR is about as likely as ordering a new 818 kit today and getting in a week or two... The WEB blab on the 3.0 also presumes the reason the 3.0 wasn't shipped with a manual during it's early years (at the least) was the transaxle would not hold up to the torque. Why a turbo 4 wouldn't tear up the transaxle leaves one to wonder of course, although I think the 6 could dump more torque from an instantaneous clutch dump than the 4, even if at continuous output ratings the torques were similar.

Figuring out the mindset of any vehicle OEM regarding parts is pretty much impossible today - unless you are an insider on the project in question. We really are relegated to trial and error, at best sharing what we know with each other, and probably going to the aftermarket axle guys mentioned above.