Used pandoc for a couple presentations recently and while it's clear the experience will be wonderful eventually, it's still a bit buggy. There is no widescreen template included with Pandoc and attempts to use other templates caused me to encounter https://github.com/jgm/pandoc/issues/5402 . The resulting .pptx files won't open in Keynote, either, but luckily they did open in Google Slides, which then let me download a visually-identical .pptx that opens everywhere. Point is, slide support in Pandoc is still somewhat new and people should not go in expecting a trouble-free experience at this point.