Start with a one cylinder. As the crank rotates, the big end of the rod rotates while the small end of the rod and piston reciprocates(moves in a straight line and changes directions every 180 degrees).
You can balance the rotating mass. The problem is the pulsing from the inertial loading from the piston and small end of the rod. This can be canceled out somewhat with the proper counterweight. The next problem is the power stroke. Every other revolution, the cylinder fires and this force can be felt. This is why your lawn mower vibrates so much.
Ad another cylinder and you have two cylinders working opposite of each other. The balance gets better to some extent. With the pistons traveling in oppisite directions they cancel out some of the inertial loading. How ever there are now two cylinders firing, so you feel twice as many pulses and these are felt as vibrations. If it is an opposed engine it will feel smoother.
Add two more cylinders and now you have an inline four cylinder and these are shakers in my opinion. The balance is great but the pulses can be felt as vibrations. Now the opposed engines like VW and Subaru and Porche are smoother because of the extra cylinders.
Add two more cylinders and now you have six cylinders firing at 120 degrees apart. An inline eight would fire 80 degrees apart. More pulses equals smoother feeling engine. From here on out it just gets better. The more cylinders that fire per revolution the less vibration you feel.
When you look at the V8 engines you can see that they are a trade off. They actualy act more like four V twins. This is terrible to try and balance. The inertial loading can't be totaly compensated for. It can be balanced, but it is a trade off and the pulsing from the firing sequence can be felt.
