As for plant capacity, the largest power plant in the world is renewable (Hydropower):22.5 GW. Compare that with the largest nuclear plant, the Kashiwazaki Kariwa Nuclear Power Plant (currently closed due to safety concerns): 8GW.
If you limit "renewables" to only solar and wind, there are already 1GW+ wind and solar PV projects operating, and more in the planning and construction phase. There's no theoretical obstacle to building 8GW solar PV projects, though it might be unnecessary, as they can be deployed in a modular fashion. There are several 500MW solar thermal projects operating and more planned.
The downsides of nuclear are not comprehensively understood. The consequences of the Fukushima Daichi nuclear disaster of 2011 may be with us for decades or centuries. The interactions between radioactive material, the atmosphere, the ocean, the human body, and other organic matter, are complex.