The problem isn't so much PLD as it's the content. All the content in the game is so heavily focused on DPS, and that was doubled down on in savage. WAR is also too good at what they do, including DPS so they are basically guaranteed in any group. And since doubling up hurts LB (and honestly that's the only reason double WAR isn't widely used) the second tank slot on endgame is between DRK and PLD, and DRK edges out purely on DPS grounds. On top of that too much of the fights cater to DRK's move set.

So that's basically the short of why PLD is in a bad spot. It's got some issues with clemency, shield swipe, and divine veil that need to be patched up, but they aren't terrible. It's just the devs forgot it existed in the current end game model.