'Can' void warranty?
I thought that while rooting 100% chance is there that it will void the warranty.
Any other disadvantages?
And what is custom recovery?
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Some manufacturer's still repair/replace even if it is rooted if its a hardware problem - its a difficult way to phrase so the best thing to say is it "can" void warrenty. Also phones can be unrooted as well and it is hard for the manufacturer to prove whether it has been rooted or not.
Other disadvantages at the moment are that google books and google videos wont work on rooted devices(who uses them anyway?

)
A custom recovery is part of the bootloader (actually thats a lie but its the simple way of putting it) and using a custom recovery you can mod your phone by flashing custom ROM's/kernels/themes etc.