Break even, assuming you could never get a refund, would be if the failure rate and average discount were the same. My experience is that the discount is >60% and failure rate is <15%.
I've never really had an issue with online refunds anywhere. That doesn't mean Amazon's experience isn't superior. Amazon's policy of refund first, ask questions later (for stuff in the <£15 category at least) means that I don't really consider the other options.
If I have a choice between buying a USB to DC5v adapter from amazon for £3.99, with next day delivery, and I know that if it's not the right cable I'll get an instant refund, or from aliexpress where it's £1.99 [0] but I have to wait 30-50 days for it to arrive, plus 3 weeks for a refund if it doesn't work, I'm just going to buy it on amazon. I don't care if I don't come out ahead!
[0] https://www.aliexpress.com/item/33051428867.html?spm=a2g0o.p...