I have been thinking about this for a while, but it only recently really clicked. Despite all the excitement about the partial declassification of Soviet archives in the 1990s, the type of information they provide is quite limited in regards to the deported peoples. Yes, we now know the mechanisms and legal structures of the instruments of repression from the inside. We also have much, much better, although far from complete or perfect statistical data on the numbers of people subjected to various forms of repression, their geographical distribution, and their use as an involuntary labor force. What we do not have is much contemporary material from the deported peoples themselves. The archives contain some letters written to officials complaining about their material conditions. They also contain police reports on the "subversive" political opinions of the special settlers. However, overall material from the point of view of the deportees themselves is lacking. There are almost no surviving diaries and other written documents outside of official state, party and police archives that would provide an insight into their lives at the time. There are some exceptions such as letters written by Russian-Germans deported as kulaks in the 1930s to relatives in the US and Canada. But, after 1937, such sources are very rare. Lots of information was simply not recorded by the authorities or was destroyed. The official archives that exist today have very little on the human lives and relationships of the special settlers.
Memoirs written after the events exist, but most of these were written long after the events when many survivors had already died and memories had faded. Oral history has the same problems as memoir literature. It would have been much better to collect this information in the 1950s and 1960s rather than have to wait until the 1990s before conditions allowed such activity without criminal penalties. Now a lot of information has gone to the grave without ever being recorded.
Ironically, it appears that despite the paucity of archival access at the time it was written that Solzhenitsyn's Gulag Archipelago remains one of the best works on Stalinist repression. Yes, he did not have accurate information on the population statistics of the labor camps and special settlements. But, he did have access to a lot of sources that no historian can tap into today. He had access to a lot of memoirs that have since been destroyed in order to protect the writers from persecution by the KGB. He could also talk to a lot of camp survivors who have since perished. This combined with his own experience in the camps and in internal exile allowed him to provide a history from the point of view of the victims rather than the repressive apparatus. This is something that nobody can do as well today.