Earlier quoted context omitted.
They said the same thing about 40G but hey, I've loved it for bridging the gap between my two (10G and 100G, respectively) Mikrotik switches. You can have a dozen Gigabit ports, as well as up to four true 10G devices on your aggregation switch, and neither would be bottlenecked by traffic to and from the backside. This has been a massive boon. However, when it comes to 2.5G, I struggle to find one good reason to use…
1x PCIE 3.0 has 8 Gbps raw speed - for 2.5Gbps duplex Ethernet you'll need 6~7 Gbps of raw link to CPU. For 5Gbps and higher, you'll need another PCIE line - and SOHO motherboards are usually already pretty tight on PCIE lanes. 10GbE will require 4x3.0 lanes
(But PCIe 3.0 of course is from 2010 and isn't too relevant today - 4.0, 5.0, 6.0 and 7.0 have 16/32/64/128 Gbps per lane respectively)