Recently I've went through the "Fundamentals of Magento 2 development" course and there was such statement that we SHOULD NOT use functions from other modules that are not defined in the API interfaces for example we should not rely on \Magento\Framework\Model\AbstractModel save() method, because it can be removed in feature releases.

Ok, fine by me, but going this way we should not use, for instance, the load() function from \Magento\Framework\Model\ResourceModel\Db\AbstractDb right? Yet, it's recommended to use it in the same course. Can anyone point me in right direction?