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I can't help that he has shit mechanics or keeps being crashed into. I've ridden around Spain with a 42T MTB cassette on the stock Di2 RD. The 1400k Italian audax we just got back from was done on the same size cassette. The fact is it works even though it's well over their specification. The missus used their cable RD and 40T and it's less good - it struggles in the highest gears, but her shifters are pretty meh so it could just be those and it's not like she's racing down hills. Anyway, I want Di2 on her bike and a smaller crankset and cassette on mine. None of this is any reason to go near SRAM.
You've seen the videos of WVA jamming his Shimano di2 chain for the 3rd time in one TDF stage?
FWIW I'm no SRAM fanboi, I have it it on only one bike (without a front mech), but bodging hangers to fit oversized cassettes etc that works nearly always is just a ballache.