There are many tools in Pixelmator Pro that are bitmap operations and therefore are incompatible with SVG and result in some ugly rendering errors. Things like clipping masks and gaussian blurs are not supported by SVG, it seems, as Pixelmator opts to entirely ignore these effects within the SVG export pipeline. What I would suggest is for Pixelmator Pro to present a warning dialog whenever the user attempts to export an SVG that contains methods and effects that only make sense in a bitmap space. This dialog, I think, should specifically call out what tools and effects will be discarded during the render stage and should perhaps even provide a preview of the resulting file.
What features would you like to see in Pixelmator Pro?