Used both, from pf to opn maybe 15months ago. Never had issues with either but I've had issues with how pf is managed and just seems to get another reason to dislike every so often.

Depends on your issues but go raise bug report with opn. If opn started to cause me issues then I'd be more likely to goto openwrt I think,rather than pf.

Fairly frequently,but no real schedule. 3wks-3months, whenever I get some time to update without it being a problem. Primarily for patching & new kernels reasons but has caught the odd disk issue where btrfs was struggling and I didn't catch when running.