The SVG specification is rich and OmniGraffle doesn’t have import and export code to cover all of it. We prioritize our SVG work partially based on user feedback and what we think will benefit the most users. Unfortunately not everything in the SVG spec aligns 1-to-1 with an OmniGraffle feature and not all OmniGraffle features are available in the SVG spec so our ability to support SVG is limited in that way too.
How can I help?
Sending us examples of what isn’t working is certainly helpful. If you include the source files and a screenshot of the expected result that gives us data to work with and lets us know what features you’re taking advantage of.