Electricity is not a source of energy, more of an energy medium, still produced by mechanical means. That translated to a source of heat to make steam, to drive a turbine, then to rotate a conductor in a magnetic field. Hydro is nice, skips the heat and steam process.
Back in the 50's and 60's, even earlier, hydro was are only source of electrical production. And was extremely inexpensive, but only a limited number of rivers and dams, that could not keep up with the population growth. So other heat sources had to be found such as coal or natural gas.
Nuclear fission could be okay taking a larger molecule and breaking it down to smaller ones, a great majority of these process or inert elements, but a rather small percentage produces what we like to call, crippled atoms that are highly radioactive. And the half life of these little buggers is measured in thousand of years.
If all of the molecules could be broken down into smaller inert elements, this wouldn't be a problem, that relatively low amount or radioactive materials is a major one.
But this is only one of the problems, comparing the nuclear core to say acres of coal, an extremely low source of energy density, that nuclear core as an astronomical amount of energy and must be very carefully controlled. Or a melt down will occur. High energy density is always a problem. Ha, use to know this field quite well, been like over 50 years ago, brain is getting a bit corroded. But these are the basics.
Really difficult to predict if with nuclear fusion will have the same problem where deuterium is suppose to be combined to form a larger helium molecule, just the opposite of fission. Crippled molecules may also be produced that would also be highly radioactive.
Population density is one of the key problems, can recall leaving the Chicago city limits and being out in the country, now solid city. Same with all the major cities in this world. One thing for sure, our technology is not keeping up with it.