This is actually the smartest thing Oda ever did, and such a FUCKING FANTASTIC use of Buggy. Turning his mercenary business into a bounty hunting business opens up so much.
-The whole world is a giant battle royal now, with everyone hunting Pirates AND Marines
-So many old characters can come back and join Cross guild
People acting like this makes Mihawk look weak are missing the point. A guild isn't a monarchy. Buggy isn't ruling over Mihawk or Crocodile: he's just the face of the Guild, who holds everything together. A guild is: "An association of persons of the same trade or pursuits, formed to protect mutual interests and maintain standards"
Mihawk and Crocodile aren't underlings: they're collaborators. You don't need to become Buggy's underling to join Cross Guild: you just need to bring in the heads of their enemies.
Anyone can join, as long as they are willing to bring a Marine's head in, without joining the organization. This is why the Marines are so fucked, and Buggy is a Yonkou now:
Bounty hunters, Pirates, Nations outside the WG, random poor people: everyone is hunting the Marines now. Buggy turned the world on its head, somehow. Now... the Marines are being hunted. I love this twist so fucking much, Cross Guild is awesome.