A hardware router is distinguished from a software router by the fact that a software router is capable of executing general-purpose instructions.
We have different definitions, and thus will come to very different conclusions based on those definitions. To me, a software router means a router in which you install the software, and thus are in some sort of control over it, as opposed to a prepackaged all-in-one solution, where you (typically) aren't in control of anything other than its configuration. If you don't configure the software yourself, the router is essentially a black box, and whether it is using hardware-assisted routing or purely software routing doesn't significantly change the level of trust.
The reason the trust level doesn't change is that it is not really feasible to have a router that is incapable of running general-purpose instructions. Such a device cannot be configured usefully, except perhaps by swapping out a configuration ROM (which would be highly impractical in most real-world environments). I've seen lots of two-tier setups, where special-purpose hardware does the actual packet routing and a general-purpose CPU runs some sort of web or SNMP interface for configuring the device, but you still have a general-purpose CPU that can be attacked, and can then be told to reprogram those special-purpose devices to route or modify packets in a different way, up to and including diverting some portion of the traffic to a port on the general-purpose computer for deep packet inspection.
Therefore, black-box hardware-assisted routing is no more secure than black-box pure-software routing. From a security perspective, the only things that matter are the extent to which the software is under your control and the extent to which you trust the software vendor.