If you want to stay sane and make it through an entire season of wagering in this league, risk 5% or less on each wager. There are enough penalties that'll make you start to dislike this product on the field. It's getting worse every year.
I agree that you can't bet an under 'comfortably,' but betting unders can still work and be profitable if you choose your spots and bet wisely. Basically, you can't just bet the over blindly and expect to crush it. The oddsmakers will adjust at some point, if they need to keep raising the totals from where they will be in the first few weeks.
Last year, based on my calculations, the O/U for the entire league was 270-238-4....which equates to a 52.7% win pct for the over. Despite the obvious points increase in the NFL due to the rules changes, and all of the publicity that gets, the oddsmakers have clearly done a good job with the lines as betting the over isn't a certain 'kill it' situation.