I'm guessing you're talking with multiplayer in mind here, yes?
Because it's hard to tell in SP with the current state of the AI, but Necrophages snowballing was a thing in EL1 as well: I don't think it prevents everyone on the map from playing an eco focused game, it's just their unfortunate neighbor(s) that are gonna be wrecked. With the amount of time conquering a single territory takes in EL2, even wiping out a single player who's expanded a bit takes a long time, so what you're mentioning doesn't sound realistic, to me.
That being said, I'm not claiming they are balanced! They do seem pretty busted, I'm just saying their science output tends to be abysmal in comparison to what you could get with any other race, so, while you conquer everything, another faction far away from you could (if the AI were able to actually grow its eco) be snowballing hard in science.