I've been using Untangle in this configuration for some time. And, while it can be made to work there are several things that are less than ideal.
1.) Pretty block pages don't work. I still get a very basic HTML block but the nice pages just won't fire despite my best efforts.
2.) Reports don't show activity on the second subnet.
3.) DHCP server doesn't have the ability to serve the second subnet without hacking the file by hand. GUI support here would be nice.
These features, as well as the ability to do IP ranges as an alias on external would go a long way to enabling Untangle to operate as a full core router. But, I will admit, I would rather multi-wan/failover was implemented first.