Nothing but a lot of people who don’t know what they’re doing use them instead of routers or gateways. This gives their devices direct access to the internet which is a major security issue but it also means that they end up using all IP adresses that their ISP will provide. All new devices that try to connect will fail to get a new address. Then they go online and ask why their internet connection is shit.
Yeah, Ethernet connectivity is a common issue in Linux. This is why it’s almost never used in servers. It’s definitely not something you are doing wrong.