I honestly found it difficult to see the dragons as any kind of legitimate threat. They're disorganized, unloyal, incredibly weak once grounded, do not make use of their "legendary" shouting powers, and their leader was an incompetent fool who missed at least three opportunities to put down the only thing that could allegedly stand in his way.
As it Implies their primary combat tactic is meant to consist of throwing themselves at the enemy repeatedly in the hopes of whittling them down due to being renewable unless slain by a dragonborn, I am not impressed.
Three things cause this.One thing is the fact that many of those dragons survived the dragon war and had to go through life until their eventual death leaderless. The whole threat is from Alduin reestablish order, which would be difficult since most dragons still remember his defeat at the end of the dragon war. Odaving seem to describe the dragons as having lost respect for alduin but still fearful of his power. Currently the dragons seem to be on their own agendas
The second is just the fact alduin is very arrogant. It is a character flaw that alot of villians seem to always have. Just kind of how it goes, he thinks that the hero could never actually defeat him until he does. Think parthunaax said dragons are generally arrogant, power hungry and untrustworthy and I'm sure their king would the biggest example.
Third is pretty much just gameplay. Guessing the devs did not want dragons using alot of the more powerful shouts since it can cause alot of callateral damage. I played with the deadly dragons mod which gives dragons better shouts and have had to reload a few times because they killed someone important like the blacksmith or merchants, ect. Once had a dragon kill everyone in whiterun.