Aluminum has less stress cycles than steel--that's why the aircraft industry keeps close track of the lifespan of various aircraft parts. Aluminum is also softer and "peens" or "coins" easier. I have had that happen to me with aluminum weaver style mounts.
Guys who shoot a lot with heavy scopes seem to be the ones with failures of rings or mounts. Back when scopes were heavy most of us used steel mounts and rings, when scopes became lighter, aluminum mounts/rings became the norm. Now that we have gone full circle there is an argument for steel.
