Mmm I don't buy that at all. Aluminum is inherently weaker and has less resistance to corrosion than powder coated steel. I wonder what company his bent ones were from. don't forget that there are many grades of steel as well.

Same reason aluminum drop hitches aren't the best for frequent heavy loads, they break down and wear.

 

Aluminum is weaker than steel, but when you compare a solid aluminum spacer to a welded steel spacer, the design of the steel spacers makes them more likely to bend/compress around the edges than an aluminum spacer. Also, aluminum is very corrosion resistant even when not painted. Like I said earlier, steel spacers are fine for most applications, but if they see heavy offroading or similar situations, they can have problems.

 

I see your point about solid versus cnc and welded. But road salt eats aluminum up almost as fast as low-mid grade steel. It can be more resistant to rust due to water and moisture though, but I still wouldn't like it. I see aluminum bend in applications when thicker than steel counterparts, and that's what sticks in my mind. It's much weaker, but it's certainly possible that because it's solid, it's fine or better.