Blackmagic batteries are notoriously bad/short-lived, so I'd get a few backups.
On paper it sounds awesome. But it is kinda overkill to shoot at 6k when most people are gonna be streaming it at 1080 max. Also the build quality on BM has never impressed me too much, they look very plastic. On the other hand, something like a GH5 has more metal on it and some weatherproofing.
The other drawback is of course, the lack of motorized zoom. Sure, there's an attachment for it but it costs an arm and a leg. So, get used to mastering the art of the tripod for long lens shots. Fluid head is the way to go.