Mermails -
Advantage - They can loop easily and swarm the field. Much more then people realize. Most ppl use Mermatlanteans, but pure mermail is so much better...
Dis - If u dark hole the field, they can easily be otked considering they need a 2 turn remake
Fire Kings -
Advantage - They can search easily, and pop the field. Cards like mirror force and torrential only slow them down, but at the standby phase, they instantly recuperate.
Dis - BTH and other cards like prison can easily kill the deck with a single swoop. And considering most deck mains them....they really need some more support.