I don't see what you are talking about of not implemented. Markdown tables is very well converted to latex. But unfortunately has some issues that isn't suited to every situations and I agree to maintain longtables as the default option, as the standard latex 'table' environment with 'tabular' has other issues such as not handle footnotes, not adjust columns and not split between pages.

If you can live with all these limitations I used in some of my documents a filter that I wrote in Python to read a table and write a tabular instead of longtables.