In the latest version of Krita (4.3.0 at time of writing this) the selection tools were greatly improved. I’ve been using them a lot in my work lately. One quick tip I discovered was that snapping is enabled on the outline and polygonal lasso selection tool by default and it helps to disable it for more acurate drawing selections by pressing Shift + S and toggling the options.
Another very useful feature I’ve used with selections is the ability to asign colour labels to layers and tell the selection tool to target particular layers with a colour label. It saves time when working as I can set my line work layer to have a colour label and then on my painting layer I can select areas with the magic wand tool quickly without having to switch between layers to do so.
The similar colour selection tool is very useful as well, especially when you have a “flatted color” layer as a reference. If you want to select similar colours and change them with the hue, saturation and value filter it can be handy.