Winforms is great until you try to make windows dynamically sized, or deal with DPI nicely. In every other regard it's still fine, and for accessibility actually _better_ than many subsequent frameworks. And produces nice small fast executables.
I assume that if Microsoft hadn't abandoned WinForms for the next thing, it would support dynamic sizing and DPI properly. It's mindboggling how much time and effort they've wasted coming up with new GUI frameworks instead of just improving on what they have.
It does, but many still think it is like using VB 6 and don't learn the additional APIs that provide that support, e.g. FlowLayoutPanel and TableLayoutPanel.
Or, unless they've changed it, hardware accelerated rendering. Winforms was based on System.Drawing, which used GDI+, which was largely software rendering. This was confusing because GDI+ was not really related to GDI, which had and still does retain some hardware acceleration support. Even basic color fills start becoming an issue with a big window/monitor.
Winforms is also .NET based, so it's inaccessible if you don't want to write your UI in and take a dependency on .NET.
transparency as well. WinForm really struggles with the idea of stacking elements on top of one another where there is an arbitrary amount of transparency or tricky shapes. Its just not worth the hassle compared to WPF.