I think I have read that heat is hard to convert to useful energy. Can someone confirm? But the, if that's true, why use heat as energy storage?

Heat is really useful if the energy form you want is heat. If you want electricity or motion it's not the best.

"What, sir, would you make a ship sail against the wind and currents by lighting a bonfire under her deck? I pray you, excuse me, I have not the time to listen to such nonsense." (Napoleon Bonapart ~1800)

That depends on what you need energy for. Converting heat energy stored during the day to heat my house at night is easy. Converting heat energy stored into light is much harder (as the other reply stated, it needs high heat and thus is a fire hazard)

It's inefficient, but still better than completely losing out on it. Better yet would be direct reuse or remote heating.

Heat can't be converted losslessly to useful energy, but converting to heat to other forms of energy is extremely common. What do you think a steam turbine, boiler, internal combustion engine, etc. do?

"Low grade heat", less than the boiling point of water (often), can be stored in a sand system and recovered efficiently and distributed later.

But low grade heat cannot efficiently drive a heat engine to produce other, more useful and transferable versions of energy, like electricity. For that you need "high grade" heat.

The reason is, heat engines do not operate on heat. They operate on heat flow. You must have a high temperature side and a low temperature side, and you extract energy by taking heat out of the high side and injecting it to the low side, through various processes. By thermodynamics, this process has hard efficiency limits of around 35%, with that efficiency heavily affected by the delta between the hot and cold side.

The "low" heat side is often a great source of low grade heat, for industrial or communal purposes, but cannot be used to run an efficient heat engine. It's already "used up" essentially.

A lot of energy generation is just "produce heat, boil water, steam moves a turbine

So in that regard heat is very useful in producing energy. But converting directly from heat to energy? Not so much as far as I know.

Nuclear reactors do exactly that

Yes. This is sometimes called "Carnot tax".

The efficiency of any heat engine is bounded by the Carnot cycle. In simplified form, it depends on the temperature differential between the cold and the hot parts. So if your hot part is at 300C (572 Kelvin), and the cold part is at 30C (300 Kelvin) then the absolutely best possible efficiency is around (572-300)/572*100% = 47%.

You see that it quickly becomes inefficient as the differential goes down. E.g. if you use 90C to store the heat, then the maximum efficiency is just 17%.

Use it directly, see my silbling comment.