The great fact to remember about the Puritans is that they preached to captive audiences, who were in Church by force of law (pre ejection). They hammered assurance down to almost nothing because they KNEW their pews were rammed with false professors. And they were undoubtedly good shepherds in their so doing.
While I agree with your basic point, do you not think that they went too far sometimes? Or is it that we just need to remember the context in which such statements were made?
Oh yes, no doubt they went too far in some cases, and no doubt some believers had assurance difficulties because of it.
BUT... I can forgive them for it because of what they faced - row after row of people who thought they were going to heaven because they parked their backside on a plank of wood for a couple of hours a week!