Here's an article on it.
Fundamentally, Necro-Minds aren't like either hive minds or necrophages.
Hive minds are early-dominators who leverage their natural growth/assembly and starbase economy to out-muscle organics, but they lose over the long term due to various limitations on things that normal organics get (trade, factions, federations, ethics and ethic-civics, etc.) They need to move early to get a dominant position.
But necro-hives can't get real use out of pop-assembly, which has a -75% penalty, and their only natural growth is of food-only livestock.
But but if you take Catalytic Converter, food becomes your alloy economy, and your livestock base can grow much faster than a mineral base. Once you start conquering, every conquest will start feeding the war economy.
But but but if you are using the necrophage conversion buildings, each livestock-to-necrophyte swings your food income by about 5, creating huge swings in your food economy unless you limit your rate of necro-conversion.
But but but you can limit your rate of conversions to keep a stable food ratio, and after a few conquests as empire growth slows down it won't matter.
Necrophages are a slow-starter civilization that trades a much tighter early-game economy for pop/economy blooming dominance that roles into the mid-game. They start with a necro-pop shortage, but over time get more and more use out of the 5% specialist bonus, while ignorring the -10% worker penalty because they can use their starting slaves/secondary species for that.
But necro-hives have to use the necro-pops as mineral/energy drones, meaning the necro-pop shortage is worsed by force reliance on -10% penalty for minerals/energy.
But but necro-hives, as gestalts, have access to Solar Panel starbase slots, which provide 6 energy each. Early game, nearly all energy needs can be met without major energy worker investments.
But but but necro-hives building solar panels have fewer alloys for an early fleet rush.
But but but but if you take Unyielding tradition tree as your first tradition, tier 1 starbases with solar panels and hydroponics bays will pay themselves back within 10 years, and greatly free up your necro-pop economy for the early game.