[ikeboy]

Wouldn't any process running as root on the computer be able to re-flash the router?

This is also an order of magnitude harder than Whonix, while I consider Whonix, Tails, and TBB to all be the same order of magnitude difficulty. (And your router's screwed if you mess up.)

This does seem to provide better security, although probably comparable to the Physical Isolation that you mentioned.

[tikums]

> Wouldn't any process running as root on the computer be able to re-flash the router?

No, because router's management interface is only available out-of-band. This is a conscious design decision to mitigate against this threat: "In order to protect the PORTAL from tampering from malware (or malicious users), it also requires a third administration interface. This can be either a serial console, or physical connection."[0]

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[0] http://grugq.github.io/blog/2013/10/05/thru-a-portal-darkly/