Quote from James49478 >>

Even  the Titan's Thunderous Bellow would not help the Tarrasque against Tiamat.  Flight gives her the ability to stay in it's read quarter away from the bellow and continue to hit it from behind.

 
For the purposes of 5e combat there is no “rear quarter”; the Tarrasque can aim the Bellow anywhere along its border.

More versatile bosses that tend to not die instantly, it hink make for funner and better bosses. but mostly it boils down to how the DM does it, how well they can flavor it and make the players have fun. The creatures are just catalysts for good DM's