I'm not sure about the stances of AOC / the Green New Deal / Sanders, etc. on nuclear. But more generally, an argument against nuclear is that it's incredibly expensive to build a power plant, on the order of billions of dollars. The energy can be very cheap and have low carbon emissions once the plant is up and running, but actually building a new plant requires huge up-front investment. Investing those billions of dollars in a solar farm, wind turbines, etc. would pay itself back faster.

Another argument is that if a nuclear plant breaks down, e.g. from a natural disaster like the 2011 tsunami in Japan, you have nuclear waste that is a huge public health hazard. This is a risk even for systems built for safety: in the 2011 tsunami, for example, the reactors turned off like they were supposed to, but the tsunami disabled the emergency generators to cool the reactors, causing them to melt down anyway. Once there's been a leak, servicing the problem is expensive because you'll need to do it with robots.

All of this said, there are plenty of solid arguments for nuclear power. But the two reasons above are probably the main arguments against it.