Aftermarket wheels...how safe are they??

Holes in the rim design serve 2 purposes. 1 to cut down on un sprung weight and 2 to help airflow around the brakes.

Here are a few common sense things to do if you want to minimize the chances of wheel failure.

Buy new wheels. You don't know the history of used ones. Especially alloy wheels. Fatigue can be invisible. And they will carry liability insurance in case of a failure..
Like aluminum connecting rods. they look great until right before they fail. Why do you think there are so many used aluminum rods out there that are cheap. You can't see if a rod has seen an 11k RPM overrev because of a broken trans or missed gear.
Only buy Street/DOT rated rims. Off road racing wheels ( pavement type racing) are not usually designed to see debris, pot holes etc... on a closed course.
The street is full of hazards and poor pavement and subject the wheels and tires to much more stress than a typical race does. Especially when used for 1,000's of miles.
Make sure that the studs and lugs are correct for the wheels and are hand torqued properly, Always follow the manufacturers installation instructions.
Make sure the rims and tires are being used for their designed purpose.
Run the correct air pressures not just what you think they should be. You are not an engineer...