Here is a brief physics lesson on the forces behind lifting a solid, stable object, such as a barbell or dumbbell, and a large, bulky, shifting object, such as a sandbag. There are a couple of issues at play here, so here goes. As a former hi-skool science teacher, I have some memory of this stuff.
The short synopsis of this article is that it is harder to lift a given amount of shifting sand than it is to lift a solid barbell or dumbbell. Why is this so? I will attempt to explain why in the language of science.
Force, or strength, when applied in a twisting motion, is known as TORQUE. Torque is a measure of the strength that a car's engine (or more commonly a truck's diesel engine) forces upon the driveshaft to twist it to force the wheels to turn. So torque is a measure of strength. Power, on the other hand, is a measure of the RATE OF SPEED at which either an engine or you and me can force motion upon some object.
If you have ever watched figure ice skating, you see that when a skater goes into a spin, they have their arms spread out. When they bring their arms in close to their body, and their feet closer together, they spin faster. This spin is called angular momentum.
Now for the weightlifting part. The bar of either a barbell or a dumbbell is a simple machine called a LEVER. And this bar, or lever, gives leverage, called mechanical advantage, to the act of lifting the weights on the end of the bar. In other words, the leverage of the bar makes it easier to lift the weights on the end. Also, the bearings in the collars at the end of the bar add angular momentum to the lifting process, making it still easier to lift the weights on the end. The leverage of the bar adds both torque and increases the power of the lift to add both strength (torque) and power (speed) to the motion of lifting a barbell or dumbbell. This makes lifting solid weight easier.
All of this leverage, also called mechanical advantage, makes it easier to lift a given amount of weight. There are lifting bars called AXLES that have no bearings on the end, and actually make lifting harder. This is a step in the right direction.
Now, let's talk about lifting a sandbag. It is a lump with little or no mechanical advantage. The advantage of our sandbags is that they have no external handles, which provide a little leverage on the weight of the separate sand. All of the other guys' sandbags have external handles which act like a lever to help lift the sand load.
Also, with any sandbag, you are having to make constant tiny adjustments with your body's stabilizer muscles to the shifting load of sand. This gives much better functional fitness benefits than lifting a solid weight.
The only downside of sandbag training is that you have no bragging rights that come with using bars to lift as much weight as physics and your muscles will allow. To this day, it seems to be the whole process of weightlifting is to be able to brag about how much you can lift in a given exercise.
Again, sandbag training is not about bragging rights. It is about making the load HARDER to lift, rather than easier. And that increase in difficulty increases the functional fitness benefits greatly. Also, the momuntum of the sandbag as it is being lifted is broken up by all of the grains of sand that shift while you are lifting it.
In short, sandbags are harder to lift than barbells and dumbbells, and give no bragging rights, but give superior funcitonal fitness training benefits for performance-based results.