OK, so I'm a materials engineer by degree, but that was a very, very, very long time ago. However, what I'd suggest is that a cast reel is likely made of a more brittle material, and may have more voids in the material than one that's cut from a billet. A machined reel will likely survive impact (like dropping on a rock) better than a cast reel (I said likely, not for certain. I'm sure someone has a pile of cast reels that they've dropped from 10K feet that are still scratch-free).

One thing to be concerned with however, is some companies are probably casting reels in near-net shape, then machining the surface to be able to sell them as "machined". These would likely have the same properties as cast reels, not machined ones.

If anyone wants to call me out on this, go ahead. It's been a long time since I did any true metallurgy, and casting/forging/machining was not my specialty.