Swing suffered from two central weaknesses: the tutorials provided by Oracle were... often insufficient. And you could pick up some bad habits by following them. And then there were the UI builders where anybody could draw up something mediocre.
Real strength were the lightweight components where you could about change everything in your Look&Feel. Hexagonal radiobuttons? No problem. Want the drop down button of that combobox a bit larger? There you go.
SWT... wouldn't really categorize that as an UI toolkit. SWT is mainly driven by eclipse, not developed independently. All controls commonly used by eclipse run reasonably well. Using anything else, especially cross platform is risky. 2D graphics is especially slow. So it's only a reasonable choice if you want to build something that looks and behaves like eclipse. There's some styling of components via CSS, but it's not complete...