I think it’s more just the general category of hardmode swords without projectiles that’s the problem than just Excalibur. Breaker Blade, all of the Hardmode ore swords, Excalibur, Keybrand, Cutlass, Brand of the Inferno, all Phasesabers. Excalibur fits into this category and is pretty useless, but it’s more than just Excalibur.
The issue with these is they are outclassed. They don’t have projectiles to make them usable in bosses unlike the Terra Blade or Influx Waver, they have no/negligible benefits on hit unlike Horseman’s Blade or Bladetongue, or they lack the DPS or disruption that makes Psycho Knife and Fetid Baghs usable.