Colorado defeated Las Vegas, 3-2, on Saturday. I mention this because that game holds importance to this matchup and is a reason why I like the Under. 
The Avalanche-Golden Knights game was played outdoors on a golf course in Stateline, Nev., which is near Lake Tahoe. That game started during the day and had to resume at night after around an eight-hour delay. The reason for this was melting snow causing bad ice conditions as there were no clouds and unseasonably warm temperatures. Players had trouble with their balance and were falling down. There was one goal scored in the first period. 
The Flyers and Bruins will meet in this same venue with similar weather conditions. Realizing this, the NHL moved the start time back from late morning West Coast time until 4:30 p.m. West Coast time. So there still should be sun and daylight for at least the first period. This is going to make the ice more sticky and slower than normal compared to indoor rinks, which the players are used to. Note, too, the site of this game is in high altitude up in the mountains so attacking players are likely to run out of breath sooner. 
Philadelphia is missing much of its offense with Claude Giroux, Travis Konecny, Jakub Voracek, Oskar Lindblom and Scott Laughton all out because of COVID-19 protocols. The short-handed Flyers have managed just nine goals in their last four games, an average of 2.2 goals per game during this span. 
The Bruins should play with great intensity having lost their last two games. They've scored just 10 goals in their past five games, an average of 2.0 goals a game. Boston will be minus David Krejci, its second-best center. He suffered a lower-body injury in Boston's last game.