To have different drop rates for different bosses would be more work for the developers and why would they want to vary them?
I feel certain that the drop rates are the same for all the named enemies. Some people claim that a particular legendary has a drop rate of 3% or whatever. In some places, people say that the rate of legendary drops by named enemies (bosses) is 1 in 20, or 5%. drop rate charts and was not able to find any consistent, much less documented, information. In early January 2018, I Googled Borderlands 2 Legendary drop rates and. It's been collapsed because it takes a lot of space and may not be of interest to everyone. This section contains a lot of technical information about drop rates. Others have features which don't add much to their power and, in fact, may not be as powerful as non-legendaries of the same type and level, such as the Emperor machine gun, the Rocket Speed Bunny, and the Gunerang. Some legendaries are so powerful they can continue to be used against enemies up to 20 levels higher than the weapon's level. The Frakker shotgun gets bad comments from a lot of people, but after trying it out, I was very impressed with its wide-range killing power. Google the weapon name for more information about each weapon's features, but if you get one, try it out for yourself.
Legendary weapons each have special powers which may not be immediately obvious until you use the weapon, so don't sell off a legendary without researching it first. The BL2 Walkthrough on this site tells when a legendary is available, usually with a reference to this page. See this Wikia page for a list of such weapons. Borderlands 2 - Legendary Weapons Borderlands 2