It was the tradition of New England churches to keep pretty good birth, death, and marriage records. Plus, the Mayflower gang and other early arrivals have been fairly extensively documented. Once you can hook your limb of the tree into the existing documents, the rest of the database is just there.

My wife is into genealogy as a hobby. Chasing my New England roots was an easy warm-up exercise. Chasing her Nordic roots through revolving patronymics, now that gets mind-bending, and actually the trail goes cold pretty fast unless you get lucky.