For the reasons you give, they are not fit for the purpose of making accurate energy measurements. However, they may, IMO, be adequate for the purposes (generally 'guidance', particularly in relation to changes) for which many people use them.The wireless types that cover the whole house such as the Owl are totally inaccurate and in my opinion not fit for purpose. The reason being is that they only measure current and assume a fixed voltage and unity power factor neither of which happen in practice.
Kind Regards, John