when you try to replace a specific block like redstone with sandstone it will work properly:
_{/fill ~ ~ ~ ~ ~ ~ minecraft:redstone_torch 4 replace minecraft:sandstone 0}
the redstone torch ONLY replaces sandstone.
but if you try to do it with comparators like this:
_{/fill ~ ~ ~ ~ ~ ~ minecraft:unpowered_comparator 4 replace minecraft:sandstone 0}
you will get an error saying it doesnt find brackets. so the correct way it doesnt give an error is like this:
_{/fill ~ ~ ~ ~ ~ ~ minecraft:unpowered_comparator 4 replace {} minecraft:sandstone 0}
but then it ignores the sandstone and replaces any block