We drove from Quebec to Goose Bay to Blanc Sablon to Newfoundland to Nova Scotia last August. I can't remember exactly how much of Hwy 389 in Quebec was unpaved -- I think about 400 miles. It was not too bad if taken slowly. Actually, we needed to go slowly on the paved protion of 389, also! But we enjoyed the experience. From the Labrador border to Goose Bay, the road was good pavement.
From Goose Bay to Red Bay (Labrador highway) was another story. I rate that road as horrible. Even taken slowly, I had some damage to both Fifth-wheel and truck from pounding. I would not want to repeat that trip.
If I were to do it again, I would return from Goose Bay the way I went up (Quebec 389) and enter Newfoundland via the Nova Scotia ferry. That's a must longer trip to be sure, but the pounding from the Goose Bay-to-Red Bay road was just too hard on the equipment. I spend more than a week in St. John's getting the truck repaired, so the few extra days spent taking a longer road would not have been wasted, had we gone that way.