It’s worth noting that HTTP/2 is only a part of the difference there—reqwest is generally designed to spend more code on better correctness and performance and

It’s worth noting that HTTP/2 is only a part of the difference there—reqwest is generally designed to spend more code on better correctness and performance and starts with async, whereas ureq is designed for simplicity and is synchronous-only, which all tends to lead to less code. For the point in question, it would be interesting to compare reqwest minus HTTP/2 support, or ureq with HTTP/2 support. On the matter of reliability: I’d honestly expect HTTP/2 and HTTP/3 to end up more reliable than HTTP/1 (my guess is that HTTP/2 probably already is, but HTTP/3 won’t be just yet), because binary protocols require more careful implementation and generally leave much less room for bad implementation, whereas text protocols are simple enough that the average quality of implementation is much lower (because barely-skilled developers can make something that mostly works without even looking at the specs).