just played around with the Shortcut Forwarding Engine on R7000 / r44406 over the 2 days with 4G USB Modem.
it seems there are still issues with SFE. ppp0 is up. internet traffic stops working from time to time.
tcpdump shows packets on all interfaces, but no routing / NAT anymore. ip tables seem to work, as dropped packets are logged.
nothing related to SFE in syslog or console log.
so far only reboot restored routing / NAT.