There are a lot of more configurable and functional devices out there, but SRM devices are aimed more at home users with simple networks than businesses with segmented networks. But then I see these at business prices.
Take the firewall function: this is aimed at an outside / inside topology where WLAN and LAN are trusted (ok, so Guest WiFi can be separated and set to access just the outside, but generally this is the case).
But for business firewalls the devices will have independent interfaces and each can be configured into its own zone/subnet. Here the firewall is a router for the connected networks and, most variants, won't differentiate that one interface is outside so the firewall policy must be defined to manage all traffic from every interface to every other. Not only that, the interfaces can support VLANs so that multiple logical connections can be presented on a single physical interface.
Having said this, the SRM devices provide a good level of functionality for home and simple business use. For Mac users of Airport devices, like me, then it's a big step up in functionality. There's bit of a step up in knowing the interface, but at least it's not tied to a Mac/iOS app.
Some have said they have issues with Time Machine support. I use an old Iomega ix-2 NAS for Time Machine and it occasionally suffers from needing the backup to be started afresh. I also use Drive for working docs (with Hyper Backup) and Carbon Copy Cloner to rotate full disk clones. TM is only used to get a very recent saved version.
