Samsung 840 EVO SSD drive experienced widespread slow-down problems and when the issue was investigated, the cause was determined to be charge leakage from the NAND cells.
Upon write, the NAND cell is charged to a particular voltage level, each level corresponding to a state. With basic SLC there are only two states, high and low. But with MLC and TLC, many more possible states are used to increase data density.
Over time the voltage leaks from the gate, making it hard to tell which state it is supposed to be.
This was the problem with the 840 EVO. The voltage of previously written cells had degraded in just a few weeks - the data was not yet lost but multiple re-reads and error correction were required to determine the correct contents of older sectors. This was perceived by the user as massive slow down on older data.
Remember this was just a few weeks. I will wager the data would be entirely corrupt in 1-2 years. So I would not use a SSD for long-term archival, definitely not a TLC model. MLC is slightly better but I would still be wary about retention after several years. SLC will last a very long time but these are rare and expensive now.