lol, I don't think your caught up with the manga. Brook's devil fruit is awesome. His new powers are even better.
SPOILER!
Brook can now separate his body to move individually from his skeletal body. During the two years of hiding, Brook learned how to use his devil fruit more effectively. He can also re-attach his limbs when pulled off. According to Brook, as long as his bones aren't broken, his core remains steady and unharmed.
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Every devil fruit has its own weaknesses and strengths. The way the user puts them to use is what matters; Luffy for example. His devil fruit is Rubber. At first, you would think that, "oh, how'll rubber be strong?", but the way Luffy uses it is pretty amazing. A user could have a strong devil fruit but if the user doesn't use it to its full potential, the devil fruit will be considered weak.
Logia type devil fruit could be considered the strongest because physical attacks will not work unless it has an apparent weakness like the Suna Suna fruit - weak to water - which would give the opponent a plan to counter the user. Busoshoku Haki allows the user to fight evenly with those with Devil Fruit powers. It allows you to land a physical blow on even a logia type user.
The Dumb Dumb no mi is the dumbest fruit and the person who gets its power will obtain an incredibly low IQ to fool enemies that you're a retard and that they can't beat you up or steal your gold because that is just wrong... So you got protection till you are the pirate king.