As others have mentioned, it's important to compare apples to apples here. Blade grinders make use of a sharp swinging metal blade that chops the beans over and over again. Grind size is very hard to control and there's no guarantee the beans will see even exposure to the blade. It'll produce a lot of unevenly sized grounds that will extract at different rates during brewing and yield an unbalanced cup. A burr grinder is much better at grinding your beans consistently to the target grind size. They accomplish this by literally grinding the beans between two metal burrs and the grind size is controlled by setting the distance between these burrs. Gravity forces the beans down through the burrs and there is little opportunity for the beans to be too fine or course. That said, not all burrs are created equal but it you go with a reputable brand (Baratza and Wilfa come to mine), you can have reasonable confidence even in their cheaper models.
Now comparing manual to electric blade grinders really depends on your application. Electric grinders will outperform manual grinders on both scales but the degree to which they do so diminishes as you scale down. In a retail setting, baristas are able to keep a full hopper the entire day because they'll go through the beans at such a rate that they don't need to worry about degassing or oxidizing their beans too rapidly. The weight of a full hopper will push the beans through the electric grinder more consistently, no room for stray beans to fly around, and you'll see far fewer fines. As you scale down in volume, that weight diminishes and you lose more of that consistency to the point where it you're brewing a single cup at a time, you won't notice a huge difference in the final product. Also relating to volume is ease it use. It's far easier to toss your beans into an electric grinder and just let them go but it you're doing a single dose of 15-35 grams then the time and effort you'll spend on a manual grinder really won't be that bad. If you're brewing for six people, you might be grinding for tens of minutes.
And because others have mentioned it, the heat coming from friction in an electric grinder will make absolutely no difference at all. The amount of energy generated is insignificant and I won't come close to the temperatures needed to catalyze any reactions in the grounds; you don't need to worry about it.