Too much voltage can instantly kill or cripple a CPU, that's why it is so important to learn what you are doing before doing it, meaning learn what voltage the CPU is designed to operate under and do not exceed that, unless you are an extreme overclocker running extreme cooling like LN2, or Phase Change cooling, or direct TEC mount, etc. (And your CPU is expendable!)
Too much heat is a CPU time killer, but too much voltage can be an instant CPU killer!
Both are dangerous. Over-voltage means bullet straight to head while with the heat the life time of the CPU is reduced. You should read and check the safe voltage threshold which the manufacturer provides in the manual. while the heat can be controlled by using coolers. Pl note that over voltage means overclocking which means more heat. In that way they are linked but both are different.