You can tag a node with code via the set_meta command. This is closer to Unity’s tag system. For example:
myNode.set_meta("type", "enemy") # set myNode type to "enemy"
var getType = myNode.get_meta("type") # get the type of myNode
The advantage of the groups system is that you can create them visually via the Group Editor as well as via code. You can also return a list of nodes in a group using the get_nodes_in_group command, which is more difficult to do using metatags.