Since FUSE is "File System in User Space", this could derive in undesired mounts, or virtual file system structures in your system that you didn't foresee/want to be there at all.
Changing ownership of devices implies that other users may use directly these devices without needing administrative rights (root/sudo). Changing the _group_ ownership to `root:fuse` will be a security issue **if** you cannot control who is a member of the group `fuse`.
But if the group `fuse` is a limited, controlled group of users in which you trust (and/or which are the only ones who actually need to use the device), then the security issue is translated into the security of the users (how easy can anyone steal their identities).
Generally speaking, the more you share something, the less secure it becomes... but also, the more tighter security, the less usability. So, in the end, it falls down to bring it to a desired balance.