Fair enough on the anecdotes,
@vman - but "no controlled per-groupset mileage table exists" was the data-quality warning, not the whole answer. There
is proper independent bench testing on the chain half of your question, so here's what it actually shows.
Chains - Zero Friction Cycling's accelerated wear rig (the only serious independent longevity testing out there):
• SRAM Eagle XO1 and XX1 chains have frankly amazing wear durability due to SRAM's Hardchrome treatment. In the CyclingTips/ZFC test, both the X01 and XX1 chains were so far ahead of any other chain on pure elongation wear that Kerin had to re-run the tests.
• GX is the trap: SRAM's GX and NX chains do NOT have Hardchrome and have really very average (poor) wear lifespan - ZFC's advice is to skip the GX chain and step up to X01, which pays for itself many times over. Same cassette, wildly different chain life within one brand.
• XO1 is also the smarter buy of the two premiums: it tested equally durable and faster than the more expensive XX1.
• Shimano:
Shimano chains tested as the most efficient on the market with decent durability; SRAM's scored big on durability. XT/XTR chains are close enough that lube choice matters more than the tier.
Cassettes - here the honest data says it's not really a brand question: if the chain is replaced at 0.5% elongation, a cassette lasts roughly three chain replacements - run the chain long and the cassette goes with it. And ZFC's testing showed the lubricant plays the most critical role in drivetrain durability, whatever chain you run.
The caveats stand - none of this was done at eMTB torque, and ZFC deliberately accelerates wear - but it's repeatable rig data, not marketing. Practical takeaway for a sub-60Nm bike: XO1 chain (even on a GX cassette), good lube, replace at 0.5%, and the cassette question largely takes care of itself. Full data's at zerofrictioncycling.com.au if you want the tables.