Bump on this thread for Clément's questions.
This feature is great, the way it is implemented also is! You only get webhooks if you're subscribed to a specific metafield namespace and if there is changes linked to this namespace. This is a great feature.
However, the fact that it doesn't trigger on a variant metafield update the same way is a shame, because this would be the best way of ensuring we are truly replicating the data in Shopify's DB. :)