Redundancy is usually discussed as a binary: you have it or you do not. The more useful question is what happens during the window when you have lost some of it and are trying to get it back.
A rebuild is not a background task. It competes with production for exactly the resources production needs, and it does so at the moment when the system is least able to absorb another failure.
The properties worth knowing before you need them
How long, at your actual capacity, not the datasheet's. Rebuild time scales with how full the thing is, and datasheets are not full.
What it does to latency while running. If tail latency triples during a rebuild, that is a production event even though nothing has failed.
What a second failure during the window costs. This is the number that determines whether your redundancy scheme is adequate, and it is rarely the number people quote.
Why it matters more than it used to
Capacities grew faster than rebuild throughput. The window has been getting longer for years, quietly, while the mental model stayed the same. A scheme that was comfortable at one capacity can be genuinely risky at ten times that, with no other change.