I agree - I've never understood this 'they always behave the same and are predictable' criticism. What are they supposed to do? Suddenly start wielding a minigun? transform into a tank? They're dragons so they do dragon stuff. Like you point out, they have different attacks.
The flaw is the game design involves regular encounters which means they can't be too difficult or else everyone would complain about that. The dragon souls/shouts thing was a very good idea and if they were going to have dragons, people would moan if they hardly ever saw them. But in true bethesda style, they went too far and made the game spam them if you e.g. fast travel a lot. IMO they should be rarer and tougher, but then everyone would complain they couldn't get enough souls. It's not that easy to balance them so everyone's happy.
I say they went too far on the Shouts front. 3 levels of Shouts? It's just an easy way of adding "holy oblivion, there's a bunch of this stuff!" feel
It'd have been more awesome if they assign different Shouts to different Special Dragons (like the very Shout you learn when you kill your first dragon). Therefore there is a justification to make them super though and lore-friendly. They painted dragons to be so awesome they break the law of physics. They could fly backwards, sure, but some dragons are weaker than the local tree-scratching bear
Those ancient dragons are seriously no joke though. They don't have a lot of HP, but the damage is ridiculous.
But I seriously think that dragons really should pay more attention to the guy that threatens its very existence instead of the newly born mudcrab