I think people only complained about the owlbear because everything else was so damn authentic. The lore, the feeling of a DnD session, the classes, etc.
And to top that of: It was just a good movie. Even for a non DnD player it is worth watching as a comedy/action movie.
Owlbears are monstrosities not beasts and therefore the druid probably shouldn't of been able to shapeshift into one (I think. I've never actually played as or with a druid)
Haven't played a druid either, but just checked the Player's Handbook, and the Owlbear is above the combat rating that a Druid can Wild Shape into as well. But hey, it's cool so i give it a pass.
Afaik using wildshape to become an owlbear would be possible! Albeit only with one specific druid subclass, circle of the moon, which allows for higher challenge rating forms, and the requirement that a druid be lvl18+. I don't think any of them were near that point lol