It's probably not a safe assumption to rely on for a device out in the field, but in my experience, OpenBSD's fsck typically works just fine unattended. I could pull the plug on my router right now and I would expect it to come back up on its own.
However, yes, sometimes a badly timed power loss will break the filesystem in a way that requires a manual fsck.
I’ve had 3 power outages this year since installing OpenBSD (which is a lot for my area, admittedly) and it has required manual intervention each time. My root FS is on an SSD, it that matters.
However, yes, sometimes a badly timed power loss will break the filesystem in a way that requires a manual fsck.