Set up a repeating command block with

/effect @a minecraft:fire_resistance 0

in it, then place a comparator behind it. The comparator should light up when the command executes successfully. Give yourself a bit of fire resistance

/effect @p fire_resistance 1 1

and observe it being taken away by the command block right away - the command block successfully executed the command. However, the comparator did not turn on.

If you run the same command on a slower clock, comparators will produce a signal.