You are mixing up FreeBSD (as you're looking at netgate/ pfsense) with its Realtek drivers and Linux (OpenWrt) using its rtl8169 driver, which are not the same and can provide different results on the same hardware. Discussing the pros and cons of different networking chipsets and their drivers can get quite religious - and even if the findings are based on facts, their interpretation is often exaggerated out of proportion or might become outdated, as drivers improve.
Comparing the old Fast-Ethernet chipsets, the situation was pretty clear with rtl8139 (and rtl8029) being significantly worse than Intel's e100[0], nowadays the modern 1 GBit/s chipsets and their drivers have improved significantly, while at the same time having become commodity hardware (the real challenges are above 10 GBit/s ethernet cards, in the 40 GBit/s or 100 GBit/s chipsets, that's where you will find real differences between different vendors and their drivers). 1 GBit/s ethernet over PCIe, connected to a fast multi-core x86_64 (or ARMv8) system is easy, rendering the quality/ performance differences mostly academic. That doesn't mean you won't find differences (and bugs) between different chipsets[1], but in general they're good enough to be used (and e.g. Intel isn't flawless[2] either) - and even if some will cry heresy, you will find Realtek ethernet cards everywhere, from cheap clients to servers and networking appliances - and they won't drop out of the skies just because of that.
Yes, if you are marginal (very low-end SOC, where every single cycle saved in hardware acceleration matters - or deal with very high data rates, starting around or above 5 GBit/s), your attention will put more emphasis on the networking chipsets, their drivers and the kinds of hardware acceleration it might have to offer, but for SOHO uses and puny 1 GBit/s lines, the differences are minimal[2].
In short, I wouldn't discard a device with 1 GBit/s ethernet on the basis of it using Realtek ethernet chips, there's bigger fish to catch.
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[0] actually Linux' rtl8139 is a lot faster and less CPU intensive than Realtek's very own, official Windows drivers, but still beaten by e100.
[1] while rtl8169.ko is a single driver, it covers at least 60-80 different hardware chipsets (from 100 MBit/s to 1 GBit/s, PCI and PCIe), all with their own little quirks and kinks, some fixed up via firmware, some not - some need longer development to squash them all.
[2] and quite some Intel network chipsets have been plagued by dreaded TX unit hangs (I've personally experienced with both e1000 and e1000e hardware, before having to patch their EEPROM), bugs with the silicon and powersaving modes, which has a potential for data loss.
[3] Looking back at their Fast-Ethernet offerings, you won't find e100 drivers in windows vista or newer (even worse for their server operating systems), but rtl8139 drivers are still alive and kicking in Windows 10/ server 2016 and won't disappear anytime soon. So while e100 ethernet cards were clearly superior to rtl8139 in their time -and did cost more than 6-8 times as much- they also lost (windows-) driver support much earlier than rtl8139; yes, this particular issue isn't relevant to Linux or FreeBSD, which still support both.
