Minecraft – Will things spawn in a monster spawner on peaceful mode on survival

As of 1.8, setting difficulty=0 is the new equivalent to peaceful. spawn-monsters=false will still get rid of the nightly mobs, but unless difficulty=0, spawners will still spawn, and your health and food will not regenerate.

BEGIN OLD ANSWER:

In order for the changed setting to take effect, the server must be restarted. until restarted, monsters will still spawn. After a restart, there should be no monsters.

The MFR spawners act much like vanilla spawners so you could take a look at the algorithm. They also require power and mob essence to function. The Tier 5 Soul Shard spawner requires nothing to run, and spawns more mobs faster.