Well, at least Banshees attack everyone within a range of 3 cells, and keep in mind that no matter how many Messengers there are, their special ability won't affect more than one. Many Banshees can handle this...

In principle, yes, and they are quite effective against summoning mages. For example: If the opponent is a summoning master (which is rare), they summon a Phoenix. You approach with your Death Messenger and kill it with its ability. It has 1,500 health, and the Banshee approaches, hits it, takes away 50 health, and it retaliates, consuming n-number of your Banshees. So, there's no need to argue that some units are bad and others are good; each unit is designed for a specific battle. I'll give my preference to Banshees because when there are a lot of enemy units, you use the ability, and it will take out many!