So I have used some aftermarket axles, with no issues, but this was years ago.
The axle complaints on the Facebook group that I have seen have all come from Festiva guys. This is because the way they mount their drivetrains, it is way out of line with the front wheels, and causes their axles to bend at a fair angle at rest.
The XR2 OEM inner joint is a Constant Velocity joint that permits a fair bit of movement. The currently available aftermarket style appear to be a tri-pot inner (despite some listings claiming otherwise?) and these appear to have a little less leeway with movement. In a Festiva application they bind or wear out quickly because of this.
So the Festiva guys are all about OEM axles, cause it's their only real option with the ridiculous way they have to set them up.
This should not be an issue on the Capri as our axles are set pretty much straight from the factory. However I don't have experience with any of the newer aftermarket axles, I've been lucky that I have not have to replace one in years. But hopefully this offers some context to the axle complaints you've been seeing.