I can safely say you haven't fought enough stuff yet, because there are very tough monsters out there. You just don't always get served them.
There are a few problems with some adventure mode beasties though, you're right. Titans made from a non-metal material are about as tough as wet tissue paper; it's a byproduct of the materials rewrite and the fact, currently, if you make a creature out of snow or sand, they're not any tougher than snow or sand.
Hydras, for example, can take horrific amounts of damage. While they don't attack 7 times with their 7 heads or whatever, the time it takes to keep them from being able to hurt people severely is long, and tedious. I fought a hydra with 17 companions, and only 7 of them survived the fight. It was totally Jason and the Argonauts up in there. Had my adventurer been the first one into the fight, he probably wouldn't have walked away from it alive.
Point is, rather than blanketly increasing all megabeast difficulty and giving them multiple attack per round (which several already have based on their speed values), it's just an issue of tweaking the specific creatures or the system (like the materials system) that's resulting in easy monsters. I prefer the fact that some mega beasts are easier because that's how the game cooked them up, rather than every single megabeast being thoroughly nasty. The issue is that some need to be built more intelligently by the game, not the fact they all just need to get souped up.