The worse plot hole for me, don't know if you could call it that, was the whole suicide Admiral Holdo thing. If you can destroy Snoke's Dreadnaught, the equivalent of the Empire's Super Star Destroyer, as well as a number of Star Destroyers with just one Mon Calimari Cruiser travelling at light speed; why then, could Admiral Ackbar not just do the exact same thing with the same type of ship against the Second Death Star in Return of the Jedi, then? Or the Republic with the blockade of Coruscant in Revenge of the Sith?