It has the worst UX. It doesn't show favicons on tabs. It has the worst selection of extensions: https://redditenhancementsuite.com/safari/. Everything just seems to be worse than Firefox/Chrome from the developer tools to extension development (you need Xcode).
To use Safari, you pretty much need to decide that performance / battery life are more important than anything else which only describes my needs when I need to milk my battery when it hits 10% with no electric outlet in sight.
As a counterpoint; I think the Safari UI is the lightest and gets out of my way. I am I the only one who actually likes the lack of favicons? Very minimalist and gets out of the way to let me do my surfing.
It does have some ux benefits too. The "back gesture" works much better than on other browsers. Firefox gives no feedback, and chrome gives very poor feedback.
Safari lets you peek at the previous page, and makes it very obvious that you are about to go back, without any performance hit.
It also comes with reading mode, which chrome doesn't. Even with an ad blocker, many pages are still layed out in ways that can only be summed up as reader hostile.
So for reading and moving between pages, I feel like it beats both ffx and chrome. So I disagree with your "more important than anything else" part, but agree that performance and battery is among its top features, and if you don't care for that, safari does become a hard choice.
We all care about about battery life performance. This comments section is filled with people like me who want to use Safari and give it a serious try every once in a while for that exact reason.
But it needs more than that when my battery already lasts 6+ hours under heavy use and my laptop is plugged in almost all day anyways.
Perhaps you can see how a reading mode that we already had extensions for in other browsers and sexier prev/next gestures damn it with weak praise.
Well, it does show favicons on tabs (with the recent update). I'm not sure what you mean "everything seems to be worse", I haven't found this to be the case. As for extensions, there are indeed fewer Safari extensions than for other browsers. I checked and I regularly use only: 1Password, Ghostery, AdBlock and Harvest.
As I said, it provides the best performance, experience and battery life.
To use Safari, you pretty much need to decide that performance / battery life are more important than anything else which only describes my needs when I need to milk my battery when it hits 10% with no electric outlet in sight.