Were they accidentally shipping them rebadged Apex groups instead of Red or something? Short of actually missing shifts, or having bearings that just don't turn, i'm not sure what SRAM could be doing wrong that would be "killing their legs."
FWIW, I have Campag and shimano on my road bikes, and a SRAM equipped cargo bike, and Campag is definitely my favourite.
For one they were forcing them to ride 48-10 cranks. Valverde has only ever used a 53-11 crack and the 48-10 was destroying his legs. There were Katusha riders confirming this with what they had to deal with for their first year with SRAM. That the gearing would destroy their legs and they'd have nothing left by the time they got to end of the race to do anything. During the lock down Valverde posted a picture of his bike leaning against his fireplace and you could more or less count the gearing and know he had new gearing. Not sure if it's his preferred 53 or if it's a 52, but it's definitely either a 52 or 53 for the big ring now and he's looked better with the more appropriate gearing.
I personally have Shimano on my road bikes. Come to think of it, I have Shimano on all my bikes. I've heard from people who use campy that it's their favorite, but it's also more expensive and I'm just riding for fun so Shimano works just fine for that. Valverde's amateur team uses Shimano.