To each their own...
Any functionality used on a phone drains battery. If you don't want it, cool. I understand some people shred through battery capacity like there is no tomorrow and they have to worry about every tiny bit of juice.
But, really, to decry the App due to battery drain is nit picking. You could say that about ANY use of the phone. You should probably power down your phone and put it in your sock drawer. The battery will last you months that way.
Most of the screen remains black. Yes the pixels are activated, but at the lowest power consumption level over the vast majority of the screen. Black costs the least amount of power. If the entire screen lit up white and red and had pulsing neon swirls dancing around that sang "Zippity Do Dah" then maybe that would be worrisome. You can set the brightness way down, you can set the amount of time the notification stays on, you can choose only those categories of notification you want to appear so it doesn't have to show each and every notification that comes about.
At 15 seconds screen time before going back to sleep, you would have to receive 240 notifications to equal one hour of screen time.
The blinking notification light drains battery too. You can color code it and that lets you know what type of notification you have, but ActiveNotification gives you more information. With the notification light, you still have to turn the screen on to check and see what the notice is regarding specifically - WHICH DRAINS BATTERY MORE as you light up the entire screen...