I think its' odd that folks want to take a relatively low, but pretty powerful ability (which essentially doubles a Rogue's HP in melee) and expand it to fit any situation that could possibly be construed to be an attack.
In my game, if there's a roll to attack, they'll get the Uncanny Dodge.
That's a gross exaggeration of Uncanny Dodge. It does nothing close to doubling hit points. It is one attack that you reduce to half damage. Not an attack from each creature. Not all attacks from one creature. One attack. It is an ok, rather weak ability that may save some hit point damage on occasion if you don't have anything else to use your reaction for like an AoO for a second sneak attack or a shield spell.
I can think of few highly damaging spells that use an attack roll. Disintegrate is the biggest damage single target spell and Uncanny Dodge doesn't work against it. Even against scorching ray it would work only against a single 2d6 damage attack. It will be somewhat useful against creatures with a high damage attack like a Purple Worm. It's not completely useless. Definitely not a particularly powerful ability. I guess for a 2nd or 3rd level ability, it shouldn't be.