Never had whoopies slip myself, you do need to smooth it out with your fingers after adjusting to ensure it isn't slack and likely to slip....
Not sure why you reckon the exped system is more robust either ?
Hi, I find the slippage is in the locking part of the Amsteel loop (as you say you can straighten some of the slack out), with some elasticity in the actual cord on the whoopie slings. After @7 uses I also found shiny friction wear on the whoopee string.
On the exped system, it's rated up to 200kg as opposed to my whoopee being rated to @100kg. After similar use the exped system shows no wear. Plus if the exped inner loops break you can make do with other ones further in - not possible with whoopies. Exped also includes tree huggers in the £20 price and you don't need karabiners, maybe a little cheaper, and maybe a hair lighter (assuming karabiners). For me personally that's a better system.
The thermarest lazy system is even tougher, but heavier, bulkier and also slips a little I find.
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