Harder mostly, See the frequency is set by huge rotating masses in the form of generators, and when the supply and demand is matched the frequency and voltage are stable, when demand dramatically increases it pulls the frequency and voltage down, which is effectively slowing the generators down as load / magnetic drag increases with current drawn. Having large inertial masses spinning actually helps smooth out frequency changes. whilst large solar farms can and do syncronise with the grid, they are reactive and do not add the same smoothing effect as humungous spinning masses.

Low Grid frequency & voltage can cause an increase in current & heating of transmission lines and conductors and can damage the expensive things, this is why these systems trip out automatically at low frequency or low voltage, and why load shedding is necessary