No monsters seem to be spawned from spawner blocks.
Steps, in creative mode (game mode: hard, mob spawning: enabled):
Create a spawner block
Hold a skeleton spawn egg and use it on the spawner block
Expected: Eventually, skeleton spawns
Actual: Skeleton avatar inside spawner block spins forever and does not spawn any skeletons
This also seems to be the case at least with zombie villagers and spiders. I didn't test with others.
I cannot reproduce this.
Monster spawns from spawner blocks have the same light level requirements as environmental spawns. When I make a zombie villager spawner in a dark area it spawns zombie villagers.