One Piece Spoiler thread (read, do not enter if you aren't reading manga)

[Aerozord]

Originally Posted by P-Sleazy

I wouldn't say they have their advantages taken away. Cause they can still still turn into thier respective element or what have you. Its just whenever their in their human form, thier body doesn't automatically do the "transform into smoke/fire!" of its own free will as it has but rather it has to be more controlled.

Also, Blackbeard mentioned that he can't transform his body like other logia users so Haki isn't gonna affect him as much. He's used to being able to take a hit as a result and his devil fruit power helps him absorb those hits hence Haki will be greatly diminished since they're not stopping a transformation, but still hitting him.

Haki negates all powers, not just logia. Its why punches were hurting Luffy so badly.