High availability means nothing if they ever shut down. The minute they shut down they must be backed by something else.
Their cheapness is a matter of quite a bit of contention too.
Name one form of generation that "never" shuts down because of maintenance or malfunction. The fact is, US nuke plants have higher availability and capacity factors (~90%) than coal, oil or gas-fired plants.
Their cheapness is a matter of quite a bit of contention too.