Not stronger, more dangerous. Buggy may not be a powerhouse on his own, but he has what Luffy does too: he makes alliances easily. People end up following Buggy one way or another, which on large scales can pose a big threat. Kuzan may turn entire countries to wastelands, but so what? The WG loses nothing. But Buggy? Buggy can turn entire countries AGAINST the WG, which is a threat to the status quo.