There is a possible issue with using a pot or kettle on a blech, although not all are concerned about this issue:
Mishna Berura siman 318 quotes the Yerushalmi that a pot on a heat source is prohibited to use because if it becomes emptied the heat source will damage the pot and the person may add water to save the pot. This does not apply to urns with spigots or pumps, as they are set up in such a way that you cannot take out the water that covers the heating element in the normal way of emptying it.
As an aside, if you will be using it on Yom Tov and want to be able to refill it, most urns have a light that turns on to indicate that it is reboiling when cold water is added. Pots do not have this issue, and the above issue is not an issue on Yom Tov. However, they make "kosher" urns with a Yom Tov switch that disables the light.