I try not making assumptions about the working status of all devices, but I know for a fact that not all cameras work on OpenBSD.
There are some cheaper cameras that lack support for USB mass storage mode, there are some that aren't quite USB compliant and fail to attach at all, even as ugen(4), and while rare.. some exist that aren't supported by libusb+libgphoto(1/2).
It's
safe to say that many devices will indeed work, and as you outlined, are easily accessible.. but it's always possible that you'll run into a device that doesn't work properly with OpenBSD.
All USB disk enclosures should theoretically work on OpenBSD as well, but, I've came across a few that are a little flaky and problematic.