I wrote to Samson Ropes about Amsteel whoopie slings asking where they would fail. Here is their response.
Using Amsteel Blue whoopie slings for suspending hammocks is a very interesting application. Thanks for the link to the thread on the hammock forum, it's a good discussion.
Recently the reduction to 60% of average break strength for Amsteel Blue whoopie slings was suspected to be a low value. Testing showed that it is actually around 80% depending on the size of the rope. On every test done the break location was at the point of the adjustable whoopie tail exit.
In collusion, the locking brummel has less impact on break strength reduction than the "sudden transition in size where the adjustable section exits the bury". If a traditional buried eye replaced the brummel, the same strength reduction would be expected because the adjustable bury exit is the weakest link.
Samson: The Strongest Name in Rope