Part of the reason this is even worth doing is that they already have the district heating infrastructure in place to pipe the hot water out to buildings to be used for heat. This storage system is basically an add-on to that infrastructure.
This is made possible by the underground district heating network that most properties are connected to. There are more than 600 kilometers of underground district heating networks in Vantaa. Around 90% of Vantaa residents live in a home heated by district heating.
So if you have a city that already has such a hot water piping network in place and has nearby natural cavesbedrock that can be easily accessed and used in this way, then the cost of implementation is low.
If you don't have the heat piping infrastructure then it's a heavy lift to retofit it into all the existing buildings. Doable, but it will take a lot of time and resources.
If you don't have the caves then this is just a non-starter.
*edit: So I got that wrong, they're apparently going to be digging (or blasting?) new holes into bedrock for the water storage. I guess you could do this anywhere where there was bedrock relatively close to ground level (I'm not a geologist, I assume it would depend somewhat on the type of rock).
I'm curious how they're going to make building-sized holes in the rock relatively cheaply. Typically every kind of earthmoving is expensive. If we're talking granite with an average density of 2.7 g/cm^3^ (2700 kg/m^3^) then they plan to move 3 billion kg (3 million metric tons) of rock.