If you have a question how an ability works and interacts with items...

...you can get a lot of information on the League of Legends Wiki.

For example; search for Ezreal and go to his page. Scroll down to Mystic Shot and expand the Ability Details by clicking "Show". It will then show you information on how the ability works and interacts with other abilities and items. Instead of wondering if it is supposed to proc spell effects (such as Luden's) or on-hit effects (such as Frozen Mallet), you can check there. This goes a long way in determining if something is a bug or intentional.

http://i.gyazo.com/70c19c03bdd3c981f6909a9f73972dff.png

There's even a Skins/Trivia tab which has interesting info such as who created the champion, and who voiced them!