It wasn't because they had poor feature adoption. It was because they'd tend to adopt features in a way that's incompatible with other browsers (of the top of my head, handling of localStorage in private browsing). It was a bit of a "fuck you" to devs since you often had to do browser detection for Safari (much like you used to have to do for IE) and have special branches or polyfills for Safari.
> What made IE the IE everyone hates was its departure from standards(?) and enforcing its own vision of the web
Apple is just as guilty of this. They crippled and bastardised a bunch of features just to push for a more native adoption (depends on how cynical you are, but some say it's for more control over device usage patterns, and some say it's for longer battery life)
It's gotten better and more aligned with other browsers now, though. But alas, I know most are just chromium clones these days