Aftermarket wheels...how safe are they??

It's like anything else. There are really good wheels out there, and there are cheap knock offs out there. There is a wide range of casting techniques and materials that yield very different metallurgic properties.

The Enkei RPF1's I run on my Duster are cast. They're also widely used in auto racing, and the design originally came from F1. Are the F1 wheels a little different? Of course they are. But I've run RPF1's on a couple different cars now and never had an issue with them. But inspecting your wheels occasionally isn't a bad idea either (not my picture or my car)
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But there are also cheap reproductions that look like Enkei RPF1's, and who knows what they're made out of or what the QC looks like.

Bottom line is, ANY rim can fail. It's really easy to slap a set of rims on a vehicle that they technically fit, but don't match up with the vehicle's weight rating or capacity. Like maybe you picked some truck wheels that would be fine on your 3.5 ton truck when it's empty, but not loaded. Stuff like that happens.

And yeah, if you hammer pot holes all day long, nothing last forever. Centerlines fail too, especially the spun racing versions. They can crack at the rivets, and if you ignore the small cracks you can have a catastrophic failure. Even steel wheels can crack, Mercedes Sprinter van wheels are actually known for this, we've had a bunch fail on our ambulances at work.