A friend of mine told me that the reason why it is difficult (for the vast majority of Swedish natives) to find older relatives than from the 1600s/1700s, when doing genealogical research, is because Russians burned most church records along the Eastern coast of Sweden. [I can't remember which year(s) he said that this occurred during, but I assume it would have to be the 1600s/1700s. History is not my expertise.]
Are you sure this is the case? I've done plenty of genealogical research for northern Europe, and I've found no difference between the records for Sweden, Norway, and Germany.
One thing that I have noticed is that both Norway and Sweden used patronymic naming for the longest time (e.g. "Bob, John's son"), which makes moves potentially difficult.
Ha, just happened to have a page open that dealt with precisely this question.
Russian raids to current-day Sweden proper were, to my knowledge, fairly rare and while some records were undoubtedly lost as a result, I suspect this problem is far more prevalent in the Eastern part of historical Swedish realm - that is, in Finland.
The major cause was the Russian occupation of Finland during and after the Great Northern War, between 1713 and 1721, also known as the "Great Wrath". During this time, the majority of parish records were destroyed or went missing, partly because of looting and partly because the Russians suspected (not without some justification) that the clergy was instigating the populace in a rebellion, and therefore punished them severely whenever locals became too uppity.
While I have next to no knowledge on genealogical research, I know that firm estimates of Finnish population, for instance, do not go beyond 1721.
Source: Suomen historian pikkujättiläinen by Zetterberg et al. (1987), p. 337.