Pixel alignment.

Lines lie about where they exist and are extremely fussy to get to work with constraints, don’t scale as they should, do not translate well in my experience to web for app work. I can see the need for using them in illustrations or in icon design (maybe) but shapes that have a defined inside and outside are always easier for me to align to the pixel grid.

For web work, it’s much easier for me to ignore them altogether.

If you’re dealing with svgs and svg styling, then for sure, you can use them, I just rarely do and have been frustrated when resizing or exporting assets.