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On the small 800 series, it's not so much the limitations of the WAN FE vs. the LAN FE, but the limitations of a LAN FE vs. the WAN FE.
The WAN FE pretty much behaves much like a routed port on the ISRs, while the LAN FE ports behave like L2 switch ports (which can often be added to ISRs).
Yes, you can often use a LAN FE port as your WAN port, but you'll find it has configuration limitations compared to the WAN FE port. For example, often you cannot configure an IP address directly on any LAN FE port.
