I’ve been pondering the same about the use of Gutenberg for websites with bespoke layouts (i. e. where editors aren’t supposed to place whatever blocks they like in whichever order they like). My take is this: whether to use blocks or metaboxes depends on where you want the content to appear eventually. If you need reusable custom content within the main content where the editors can decide themselves how many blocks they want to insert, you’d use blocks; if you need custom content in areas outside of the main content and/or want to restrict the liberty of editors you’d use metaboxes.
