The Lions forwards coach, who holds the same position with England, was critical of some refereeing decisions after Wales' RBS 6 Nations winning victory over the old enemy.
But, having seen Wales' stars at first-hand in training, he can now see how Wales claimed back-to-back Championship crowns with the comprehensive Millennium Stadium victory.
"They were very good that day in Cardiff and I can see why," Rowntree told the Western Mail.
"They are very impressive physically. Having seen them work in the gym, I am very impressed with their strength, particularly the forwards. Their backs aren't that small either, to be fair.
"They are an impressive bunch and they are very well-coached. I have known those coaches a while and they have been exceptional at their jobs for a fair time.
"You can see why Wales have been so successful."
Rowntree also echoed former Lions skipper Paul O'Connell by paying a glowing tribute to Lions captain Sam Warburton.
The Wales and Cardiff Blues' flanker is the famous tourists' youngest captain in 58 years but the scrum guru admits he is already doing a sterling job.
"Sam is very good. He has spoken very well," said Rowntree.
"It's not just about what he says, it's about what he does as well.
"He has led well in training, spoken well when appropriate, so I'm very impressed with him so far.
"He will get better the more he works with the former Lions captains in the squad. The more he learns off them, the better he will become.
"He's an impressive young man."