Lets revise. A long sliding tenon is the simplest and easiest to make. Glued only at the middle it allows the table to expand and contract laterally without issue.
The problem is if the tenon is short then the joint is weak if pressure is exerted downwards and you have little space to insert dowels. If you make the tenon longer then you make the mortise side weak and it can bow in the middle. I encountered all of these problems, though minor, with the blackbutt shaker table.
The way this is overcome is to have a series of longer tenons (which provide strength to the joint, and allow room for the dowel) and a long shorter tenon (such that the entire length is supported.)