All the Linux routers I've used utilize Endpoint-Independent mapping with Address- and Port-Dependent _filtering_.
This means you can still establish direct P2P connectivity behind a Linux-based NAT device with users behind other Linux-based NAT devices. The only time it becomes an issue is when attempting to communicate with users behind NAT devices that do Address-Dependent _mapping_ or Address and Port-Dependent _mapping_. Some *BSD-based NAT implementations are this way.
Endpoint-independent _filtering_ is only a good idea for CGNAT implementations. Having an EIM/EIF NAT/firewall setup without additional firewalling makes it possible and easy for devices to run public-facing UDP-based servers without anyone's knowledge. With EIM/EIF, once you create a NAT mapping, so long as you send out periodic keepalives, _any_ IP address with _any_ source port can make unsolicited connections to a server that the NAT mapping points to. The best compromise is Endpoint-independent mapping with Address- (but not port-) dependent filtering.
This means you can still establish direct P2P connectivity behind a Linux-based NAT device with users behind other Linux-based NAT devices. The only time it becomes an issue is when attempting to communicate with users behind NAT devices that do Address-Dependent _mapping_ or Address and Port-Dependent _mapping_. Some *BSD-based NAT implementations are this way.
Endpoint-independent _filtering_ is only a good idea for CGNAT implementations. Having an EIM/EIF NAT/firewall setup without additional firewalling makes it possible and easy for devices to run public-facing UDP-based servers without anyone's knowledge. With EIM/EIF, once you create a NAT mapping, so long as you send out periodic keepalives, _any_ IP address with _any_ source port can make unsolicited connections to a server that the NAT mapping points to. The best compromise is Endpoint-independent mapping with Address- (but not port-) dependent filtering.