As I understand it, the general rule of thumb is the SWL is 20% of the minimum breaking strength for most applications. We should not really be using breaking strength to work out how these should be used safely. And that also assumes no other negative factors such as sharp corners which do not just cause mis-shaping of the item resulting in lowering its strength, but could also physically damage it.
I think these soft shackles need to be used well within their SWL. The steelies have been around for ages with literally millions in use in various applications and very few failures (of decent rated ones) when used within their SWL. I hope history proves the soft shackles to be (at least) as good.