This may depend on your field of research. In my field, it is often considered easier to publish in special issues, because in general
all contributions are submitted to review en bloc, so there is a chance for weaker contributions to be 'kept afloat' by stronger ones
special issue editors have incentives to be lenient with rather negative reviews on individual papers, because they want their special issue to be published and therefore need a sufficient number of papers to survive
the competition for slots in a special issue may be lower than for slots in a regular issue, due to the narrower topic
papers whose general contribution may be questionable can still work in the context of a special issue with a more narrowly defined topic.
It is not possible to give a reliable estimate for acceptance rates, as this depends on many factors; however, it is likely higher for special issues compared to regular issues of a given journal.
As for the call for papers, the chance of your paper being accepted depends on whether the special issue editors seek to fill an empty slot in the proceedings of an earlier workshop, panel, etc., or whether this is an open call (and then of course on the quality and fit of your abstract/paper).