I think it was estabished that the Borg had evolved a attack against it, so there was no point in deploying it across the fleet.
Unless they shared it with the Typhon Pact, it very much would be of use fleet-wide.
can pretend it's useless and invent excuses all he wants (I'm sure they have a technobabbulant way to vent that excess heat). "Endgame" didn't show any of the cons described. In the episode itself it was portrayed as the ultimate, virtually-impenatrable defence. Just another of Trek's one-off advances swept under the rug.