I don't know that it *is* more likely to catastrophically fail. Just make the landing zone on one of the more reliable drives, or better yet mirror the landing zone with RAID or ZFS. That way your landing zone is redundant, and the only way you could realistically catastrophically lose a fresh file is by having exactly the wrong two drives fail within at most a few minutes of each other.
I've been personally working on switching my own file server over to greyhole (from ZFS) this week. I'm doing it because instead of defining redundancy on a per-device basis, it lets me set redundancy on a per-directory basis. In the end that will let me make a more thorough use of my set of drives and their mismatched sizes. I have a relatively small amount of files that are actually very important and would be genuine problems if lost.
With greyhole I can mirror those truly important files across multiple drives for redundancy (and even send them off site for super safety), while all the MP3s don't need the same kind of redundancy. If I lose them I can just download them from Google/Amazon again, or rip them from CDs again.
I think the main draw of greyhole is that flexibility in how the redundancy is handled. It lets you make the most efficient use of your drive space, as long as you have a similar situation, with files that have drastically different redundancy needs.
Another thing I enjoy about greyhole is that its failures won't be as catastrophic as RAID or ZFS. Since it's dealing well above the file system all your files are still just files. Even with zero redundancy if a drive fails the entire pool doesn't drop dead, you only lose whatever files happened to wind up in that particular spot, and all the others are still safe.