Dead of Winter - Too easy with no betrayer/co-op?
Yes, Dead of Winter without a betrayer (especially without hardcore objectives) is easy, even extremely easy with 4 or 5 players. Even so, hardcore versions of objectives can be difficult if you play them with 3 players (and some are impossible with 2 players, literally impossible).

While it might not be the case for your group, I'd argue that Dead of Winter can be very easy if there is no betrayer but there is an alpha player. Without a betrayer the alpha player can easily convince the group what to do and essentially win the game solo. This is a problem that most co-ops have, and this is exactly why I always play with the betrayer.

You also might want to reread the rules, or at least watch a "Watch it played" video to make sure you aren't skipping any vital rules by mistake.