I think the even nature of the team is kind of being overstated here. Yes, the third line is good and the 4th line plays its part, but I think because few teams have a bottom 6 that is that good, they get overly praised and the true stars get short shrift. This really shows up when you look at possession numbers:
Brighter blue is better Corsi, size of bubble = time on ice, the position on the graph indicates quality of competition and offensive / defensive starts.
As you can see, the Bergeron line and Eriksonn really stand out, and Chara, and Merlot struggles, especially Campbell. Comparing across teams is a little tough, since one player on a bad posession team can only do so much, but Bergeron has the 3rd-best CF% in 5-on-5 in the entire league (Marchand 5th, Smith 10th) and his CFrel (CF% compared to his team when not on the ice) is the best in the NHL (Marchand 8th, Smith 24th).
I don't point this out to disparage Merlot or anyone -- no one's 4th line is very good, and they have value -- but more to point out how great Bergeron (and Marchand) is. Consistently facing the best of the other team, PB's line continuously drives possession, and rarely lets in goals. They also make the KIL line and the 3rd line better by virtue of getting better matchups, though those lines could do well on their own. A 6.5m cap hit is an absolute steal.