The problem with trying to use a smartwatch as a standalone ( with sim card etc ) is the very limited battery life that these have. A smartwatch that works through the phone ( not using a sim ) has a battery that only lasts up to a day typically, trying to also support direct cellular talk/text would cut that down to hours.
Using an AndroidWear ( AW ) compatible smartwatch is probably the best way to get the most out of any such technology. Because AW is the defacto standard on Android, other apps have been developed to work with it directly allowing more functions on the smartwatch. Generic smartwatches typically don't use AW and because of that, only can work with the apps/functions they were designed for and nothing else.
Most of the Samsung watches are much like the generics, they use Samsungs proprietary OS called Tizen which again limits the apps that they can work with.