In principle, yes, but against summoner mages, they are quite effective.
For example: Let's say the opponent is a master summoner (which is rare), summons a phoenix, you approach with your Death Herald and kill it with its ability. The phoenix has about 1,500 health, and the Banshee approaches, hits it, deals 50 damage, and it retaliates, dealing minus n damage to 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 personally prefer Banshees because when there are a lot of enemy units, you use the ability, and it will take out many of them!
TE]
And if the situation is like this – the enemy has summoned phantoms (maybe several, but not far from each other), then the Banshees calmly approach, use their scream, eliminate the phantoms, and also deal damage to regular creatures. But what would you do if you had Death Heralds instead of Banshees...?