Only a few, and they're not deal-breakers.
First, bamboo rots very quickly.
This will make for a bed that gets a lot of
carbon and life fast, but also one that will be out of woody matter in a few years., then that great sponge effect from the
wood is lost, and you just have a hill of nice soil. Could be worse, could be better.
Second, bamboo whole, unsplit, is hollow and sealed.
It'll be almost like making a hill of balloons. As it rots, the bamboo will collapse and your hill will shrink fast. If it's in a trench instead of a hill, you may end up with a sunken puddle instead of a garden. Splitting the bamboo would keep this from being much of an issue.
Seems like it could work on paper. I'd suggest using tree woods and bamboo, rather than just bamboo, and either splitting it first or using very small culms.