Application with ttn-handler-EU & ttn-router-US-WEST gateway...seems to work, but

We are running an application with Handler ttn-handler-EU since most of our devices are in Europe.
A gateway ttn-router-US-WEST for USA end-nodes in the US. It seems to work fine.

Can this create communication problems with sub-band settings on the gateway and end-nodes?
What are the potential problems? if any?

Or is the Gateway ttn-router-US-WEST setting the only thing that matters for stable end-node communication? (does the gateway router setting override the TTN handler as set in the application?)