Minecraft wiki entry for Combat, associated text: Just as dangerous to the player, usually deadly if water is not near. To avoid this downside drink a potion of Fire Resistance.
Will cause anything flammable nearby to catch fire.
Takes long time to flow (unless in the Nether).
Destroys all items that come into contact with it.
Can take a long time to disappear, potentially blocking the escape route.
Does not yield experience.
Since Nether mobs are fire-resistant, lava will not harm them, this also apply to boss mobs and vexes as well.