The memory on a given card will only work well at a maximum given clock speed. The highest clock speed it can work at differs on each card because the hardware is not exactly the same. The manufacturing processes used to make memory and chips are designed to make them work at a certain clock speed, and if they work at a much higher speed, it is just luck. I have two GPUs that are the exact same make and model, for example, and one can be overclocked +1400 while the other one will only tolerate +750.
When the memory is overclocked too much, the card will either crash, return some percentage of bad data, or slow down due to errors. It's actually nice when you have a card that just has a slower hashrate when overclocked too much--that makes it very easy to tune to the optimal clock speed. If you have a card that crashes when overclocked too much, it takes a lot longer to find the right clock speed because it may run OK for a few minutes or even hours before problems occur.
For the MSI Afterburner question, you have to enable voltage control in the settings. If you have done that and the voltage controls are still disabled, it is because the GPU doesn't support voltage control, or Afterburner doesn't support that function on that GPU. I have a laptop with an RTX 2070 and voltage cannot be controlled with Afterburner on it.