But there are best practices for passwords, and those reduce the risks; and most attacks need privileges and access to the machine, which again reduces the risk.
If you're worried about stormtroopers kicking the door down and squirting liquid nitrogen on the RAM you probably have enough money to have very strong perimeter defences.
The bits stored in DRAM remain readable for a time (sometimes minutes) even after the power is cut. In normal operation frequent refresh is needed to avoid decay the decay doesn't happen nearly as fast as the refresh cycles make it seem. Cooling the cells lengthens that time, from minutes up to hours (depending on the temperature), permitting an adversary to read them without much time pressure.
I would be slightly surprised if national police use such raw methods as liquid nitrogen. It is fast, but it has risk of damaging the ram.
Much safer (but slower) is to hook directly into the bus and communicate with the ram itself. I guess its a trade off between speed and security, which mean it depend on the case specifics.
It freezes the circuits and prevents them from being discharged. When removed from the fridge, the attackers have some extra time (while thawing) to read the status of the RAM.
But there are best practices for passwords, and those reduce the risks; and most attacks need privileges and access to the machine, which again reduces the risk.
If you're worried about stormtroopers kicking the door down and squirting liquid nitrogen on the RAM you probably have enough money to have very strong perimeter defences.