Xeviat
Dungeon Mistress, she/her
You have a paralyzed target say from hold monster or hold person, all those melee attacks are criticals when they hit. So instead of the fighter getting 3 attacks that do 4d6+15 (greatsword, 20 str, gwf feat) you would get 3 attacks that did 4d6+45 each, more if they had buffs, magic weapon, or other things that add static bonus.
Ugh ... 15 x 2 is 30. I'd probably not double GWFing's damage bonus, so my thoughts would be 4d6+20; it's a boon to Fighters, but a minor bonus to rogues, less of a bonus to paladins, rangers, and warlocks ...