I've heard a lot of people say it works for them, too, but I know of enough people who had the same experiences as me that I thought it was worth the warning. In my experience, the guineas just . . . well, I suppose it's just that they thought the chickens were one of the gang. Guineas have this hierarchy that's a lot more intense than the bickering between chickens I've encountered, and the ones at the top will assiduously deny the ones at the bottom access to food and water. It's not really an issue for the guineas, as they'll just wander down to the tank to drink or hustle up another food source, but my chickens are much less capable of adequately feeding themselves if some bullies are keeping them out of the food. That can be breed dependent, but my breeds at the time were big, floofy softies. This was by far my biggest problem back then. The chickens and guineas I have now who weren't brooded together share the feeding area no problem.
I don't know if you've ever been bit by a guinea or attacked by a rooster, but I'd rather have a rooster attack me a million times over, even though my guineas have only ever bit when I picked them up to doctor them. Guineas are quick and strong, with a beak that's hard as a rock and sharp as a razor that they dig in with and twist. I'm kinda amazed they're as prone to predation as they are. Anyway, roosters don't stand a chance if a guinea decides to kick his dupa, and a guinea suddenly deciding to mate with a hen was an unpleasant and very bloody affair. The birds I have today don't acknowledge the other species beyond some effort from the guineas to give the chickens a wide berth, and it's overall so much less stressful a situation that even though I only brooded them together once in what feels like a lifetime ago, I have no desire to experiment with brooding them together again.
For the record, brooding experiences with other species have led me to only brood like with like. Brooding my geese with muscovies was a disaster for me, in that my drake decided my geese were fair game for mating and fighting, which caused a lot of problems I hadn't anticipated. Ducks and geese are another pair of species commonly brooded together, so I may just be unlucky when it comes to mixed brooding.