1. A Warden is not a boss. It is an environmental hazard.
2. Beacons are not all that useful, at least compared to the items that other video game bosses give you.
3. The Ender Dragon gives you a decorative souvenir to put on your mantelpiece. When compared to a different endgame boss, Moon Lord for example, he gives you the most powerful armor material in the game, two of the most powerful swords, the most powerful yo-yo, the most powerful spells, the most powerful sentries, the most awesome minecart, and if you are playing in expert mode, a snow globe that literally allows you to flip your gravity.