We call them 'rag' joints, but in reality they are cut out of used tire sidewalls.
Some are from steel belted radials, some from nylon, some double belted, you get the idea, they are all different.
How well a rag joint sprocket works depends a lot on the wheel.
If the sprocket fits just right over the hub you get the best alignment and with only fractions on an inch lateral movement room you won't have any 'catastrophic' failures.
If you center the 9 bolt sprocket bolts between the spokes and use the 5-4 back plate combo the rags comes together and form a sort of cushioned metal reinforced second outer hub for the spokes.
I have not tried this but I wonder if slowly spinning your wheel with a torch on the rag joints might even get those pieces of tire rubber to bond even better.
On the other hand take a Huffy Cranbrook coaster brake wheel, you have to leave the outer rag out, cup the sprocket out, and mount the sprocket right against the spokes at the hub connections or some other concoction 'fix'.
One other note is check that your sprocket is true on a flat surface!
You'd be surprised how many have some warp and will always have wobble if used.
I'm fortunate as I build a lot of shifters so there are always spares around but that is not so for first time builders and if you start out with a warped sprocket it all goes down hill from there.