not exactly.
it depends on what chipset is powering the ethernet port. Many mobos come with realtek powered nics (network interface cards) and a large chunk of those give problems.
there are some mobos that come without problematic chipsets like the ones powered by an intel nic (an intel mobo can still have a realtek nic)
honestly i think your 6th gen i5 might still have a good enough time transcoding videos in plex. GPU transcoding is supported but the main reason we try not to use gpus is a mix of factors but can be boiled down to cost.
1) price of gpu
2) price of electricity - even at idle a gpu will be using a decent bit more in electricity and that adds up.
i say give it a shot with your an igpu and if you really feel limited add the gpu later.