OpenBSD's FFS can be told to be fully synchronous, making it about as robust as can be for not being journaled. If you lose power it will lose the specific data currently being written to disk. It won't lose other data, because it doesn't keep a bunch of it sitting around in a RAM cache while lying to the writing process about the I/O op already being completed. A journaled file system, too, will lose in-flight data that has yet to land on disk if the power is cut.

I've had my share of sudden power cuts hit OpenBSD during the 20+ years I've been using it, and I have yet to ever see its file system actually go corrupt.

A sincere question: what important data does your router frequently and busily need to persist? If you don't have a UPS for it, perhaps it isn't too important after all. Maybe we have different perspectives of what a router is and should be. I manage a router (with OpenBSD) and it can drop off the power grid at any moment without losing anything of importance. If your router is in fact a multi-functional server I can understand the notion. Consider making all of the file systems fully synchronous. It has worked great for me for two decades.