Actually Flash/memory drives are sensitive to radiation. Long term storage without regularly accessing the drive can lead to situations where blocks go bad beyond the ECC/CRC capabilities of the drive to fix. If you intend to store valuable data on memory devices for the long term you should (a) use multiple redundant drives (b) use a file-system with block-level ECC/CRC error correction and redundancy (like ZFS) (c) write each block to the device twice in different location (i.e. an mirror on the drive).
The future of Flash memory is such that unless they extend the ECC/CRC capabilities of the controller, the susceptibility of these devices for radiation will increase when the cells get smaller.
In case anybody doubts the impact of radiation on electronic devices, here is an interesting experiment you can do: take your digital camera, put the lens cap on and do timed exposure with increasing exposure times (1,2,4,8, ... seconds). Then analyse these pictures for bad-pixels, or better, subtract the pictures from each other. The random bits scattered around on these frames are impacts of cosmic rays. Now apply the same principle on memory devices with much longer exposure times...
To cut my somewhat rambling post short: use memory devices as long term storage? No. Not without thought about the required data reliability.