Neat, thanks.

Can a solar sail get something from Earth to Mercury?

The issue is less that it’s the hardest to land on and more that it’s the hardest to get to, to arrive at and orbit. It takes less fuel to get to Pluto than it does Mercury.

It’s expensive and the conditions are harsh.

The daytime side gets hot enough that a rover would be difficult to operate for long. You’d also be getting big swings between daytime hot and nighttime cold, so thermal expansion would probably be annoying.

Then it’s unusually expensive because orbital mechanics make it very difficult to approach the sun. We’re currently all flying sideways with respect to the sun, so if you launch something, it just wants to continue that orbit. In order to get closer, you’d need to shed most of that momentum, which takes a whole bunch of energy since inertia in the vacuum of space just means everything keeps going forever.