They work well on multi-hitters.. and they do only offer a chance percentage of applying sleep. They won't work every single time.

And sleep can't be applied if the enemy has the Immunity or Perfect Immunity buffs active on them.

Look for characters that get advantages via sleeping enemies... Like Phantom Eddie. If he hits a sleeping enemy he does more damage....

And for true effectiveness you need a team with attacks that "don't wake enemies from sleep". You may put an entire enemy team asleep one hit, then the next hit wake them all up.