Good candidates: logos, icons, badges, stickers, flat illustrations, cartoons with solid colors, line drawings, signatures, stencils, QR-code-like graphics and pixel art. These have few colors and clear boundaries, so the SVG looks like the original and is usually small.
Poor candidates: photographs, screenshots with lots of text, images with gradients, shadows, glows or soft brush strokes, and anything heavily compressed or tiny. The SVG either loses the subtle tones or needs thousands of paths, and it can be many times larger than the source image.
When in doubt, try the Logo preset first, then compare in the preview. If edges look blobby, lower the corner threshold. If the SVG has too many tiny shapes, raise the speckle filter or lower the color precision. If the image is really a drawing, switch to Black & white.