Is a down-alternative comforter good for allergies?

Yes, that's the main reason to choose one. Down-alternative fills are typically polyester microfiber or bamboo viscose, which don't carry the dust mites, dander, or feather allergens of real down. Most picks here are labeled hypoallergenic and OEKO-TEX certified, and several of the bamboo-cover options are specifically marketed to allergy sufferers.