The reason OpenWrt does use tmpfs (RAM) for log files is because onboard flash is more valuable than some
external attached log storage.
Logging to a remote server or a USB/GPIO attached (exchangable) storage are solutions too.
However - you can check the Data Sheet of the flash used in your router. These datasheets should be available on Manufacturer or some reseller sites for popular chips.
the popular M25P80 lists in "Table 11: Data Retention and Endurance":
Min. 100,000 Program/Erase cycles per block.
Data Retention of Min. 20 years.
So even if you overwrite the same blocks with log data ALL of these block might (!) turn bad after 11 years if you write every hour (1 year=8760 hours).
(data sheet m25p80)
Your Flash question will expand to
- how much do you trust the manufacturing quality of the whole router (soldering all other chips), environment (near lightning strikes, extreme temperatures) to last ~11 years ?
- if your log is "critical" what is your plan regarding other hardware faults that tend to occur more often (ageing: soldering, electricity: capacitator quality / voltage quality)
Notice that NAND flash is far more likely to go bad - even from reading - than commonly used NOR flash.