According to Dropbox, it has performance implications and is complicated thereby introducing a large attack surface. For some reason, they believe their own kernel module is less complicated and easier to reason about than FUSE, despite having more in-kernel code and not being open source; I would venture to say their words about security are more about control of the attack surface rather than any actual implications about the quality of the FUSE code.
According to Dropbox it is okay to circumvent security mechanisms of the operating system (https://news.ycombinator.com/item?id=12463338) unless the media discovers your shady way of tricking the user into granting you full system access.
Their lost a lot of their credibility in my opinion.