What espionage equipment could China put in Cuba that they could not put inside the US?

For starters, actual spy stations away from the US, inside the borders of a friendly country? Mar-a-Lago is just next door to Cuba.

Setting up reliable and fully controlled spy networks within the borders of a hostile country is hard. Like casting Idris Elba as James Bond (to borrow Trevor Noah's quote).

With a few in Cuba, they've got proximity to develop HUMINT, gather OSINT and SIGINT, with potential tracking of US military movements in the Caribbean and Atlantic at super-low latency.