Sorry the long time without answers.
The next development version release has a better import of SVG. Not everything is supported yet: text objects are one of unsupported stuff. But you can convert them to path before saving the SVG file.
Basic SVG tags not supported yet: <text>, <image>, <filter> and <use>.
The currently released Synfig versions (1.4.4 and 1.51) don’t support <circle>, <ellipse>, <line>, <polyline> and have bugs in some <path> (mainly arcs). Outlines for rectangles shapes aren’t imported (they work fine for rectangle paths). There are some issues with fills/outlines that will be solved in next development release.