You're right, some of the 2nd half lines seem like gifts. But who knows. I'm sure the books know what they're doing...right!?!?
With totals, I don't think you can always bet the opposite of the 1st half. It depends on the sport, it depends on the game situation. For example, the Kent and St Bonny game. They scored 64 in the 1st half, o/u for the 2nd half was 76.
Kent was winning 39-25 @ half, and St Bonny didn't have their best player due to suspension. Given this info, I figured that A) St Bonny didn't have enough offense to score lots of points and stage a rally, and B) Kent would maintain a double-digit lead, so I wouldn't have to worry about 13 fouls in the final 2 minutes. Sure enough, they only scored 56 points in the 2nd half. Kent won by 28 points.
That is a MAJOR factor when you bet the 2nd half in college hoops. How many points/fouls do you expect in the final 2 minute?
Also a factor in the NBA, but not as much. With the 24-second shot-clock, teams aren't forced to foul as often.