The Spawn creator told Digital Spy that work on the project is ongoing despite multiple delays and no new issues currently solicited.
"We're limping through it," he said. "It was one of those things where everyone wanted to do it, and I was the one going, 'Guys, you're stepping into a battlefield here. Be careful because you're trying to co-ordinate so many moving parts'.
"It's coming along. I've got some pages, Marc [Silverstri]'s got some pages. It'll get done eventually."
Image United is a joint venture from Image Comics co-founders McFarlane, Silverstri, Erik Larsen, Rob Liefeld, Whilce Portacio and Jim Valentino. It is written by Walking Dead scribe Robert Kirkman.
The six-issue miniseries sees heroes from Youngblood, Savage Dragon, Witchblade and The Darkness uniting to battle the original Spawn. Three issues have been released to date.
Image United #4 is yet to be solicited.
> Read our Kingdoms of Amalur: Reckoning interview with Todd McFarlane