Where did you get that from? I've never heard or read anything like that (in e.g. books/course on geneaology/släktforskning) Rather the opposite - that geneaology here in Sweden is relatively easy among because we had fairly meticulous church records from the 17th (or even 16th) century forwards, and no large scale invasions in that period.

The Russian raids occurred under a limited time (mainly 1719-21) and in a relatively limited region (the coast of Roslagen and the northern coastal towns). So most of the population weren't affected at all. I've seen nothing that indicates the Russians would've gone out of their way to burn churches, but rather the opposite (In Hamrånge 57 farms were burned, almost the entire parish, and the steeple but not the church) In places where churches were destroyed (e.g. Öregrund), the fire spread from other burning buildings.

Now you can't burn whole towns to the ground without records being destroyed, and you might be out of luck if your ancestors lived in the repeatedly-burned Umeå in the 1700s. But Umeå was also tiny at that time, as in fewer than 1000 people.

So in short, genealogy among Swedes is usually considered easy relative most of Europe. The Russian raids didn't affect most people and didn't destroy most church records where they did. However, it's possible it may be the single biggest problem a Swedish genealogist might run into, given the lack of other cataclysms. But it's still not much compared to the continent. Just the destruction Sweden unleashed on Germany or Poland in the 17th century alone is easily many times greater than what Sweden has ever received itself from foreign invaders.