This approach is problematic because, as far as I know, all implementations expect to deserialize from a complete buffer -- at least all official implementation

This approach is problematic because, as far as I know, all implementations expect to deserialize from a complete buffer -- at least all official implementations. That means the entire "stream" would have to be buffered in memory. You can, of course, simply read field by field, but few libraries expose the ability to do that simply. And the naive operation then becomes quite problematic indeed.