The head space in a bucket is far too much for secondary. In a carboy, you have maybe an inch wide area that is open. You still have a bung, but it's about an inch wide where there is a possiblity for oxidation. That's really good, and you won't taste any oxidation. The headspace for a bucket is as wide as the bucket- not good at all. Even if you fill the bucket to the top, the headspace could be 12 inches across, and it will ruin the mead.