Those tools optimize for AI presenters and rebuilt scenes. Slidify.io is slides-first: faithful to the deck you already use for onboarding and compliance, with a web player learners can treat like video. Use avatars when presence is the point; use Slidify.io when the slide is the point.
Synthesia, HeyGen, and Colossyan optimize for AI presenters. They do not “require” avatars, and they can use slides as a scene. When the job is welcome or direct address, that path is the right category. When the job is a finished SME deck, keep the slide as the visual.
Mix when it helps: avatar for the welcome, narrated slides for the dense middle. Full comparison: Slidify.io vs Synthesia, HeyGen, and Fliki.