There're some pure cloud-based solutions that integrate with customer existing AP to provide captive portal and Wi-Fi marketing services. These solutions rely on AP to redirect new/unauthenticated users to the cloud-hosted portal for authentication.

A huge risk of using cloud portal for large dense venue is the performance impact for AP. Because for cloud portal deployment, the APs have to redirect each new user request to external portal and perform hotspot session controls, which severely increases AP loading and jeopardize its ability to effectively connect user device and impacts wireless stability. And the performance/latency of the WAN connection also can have huge impact to user experience, eg. very slow portal loading and authentication, especially when the link is congestion. 

With on-premise RansNet HSG deployment, the HSG built-in hotspot controller offloads AP processing and hosts local captive portal server. The local portal server delivers best portal & authentication response time since the portals are served "locally" off from the on-premise box. Most importantly, the HSG hotspot controller takes away AP hotspot processing with a dedicated high performance appliance, to issue device DHCP IP, authenticate new connection requests, track each authorized connection and perform bandwidth/session control, etc etc, so that AP can optimally perform advanced wireless functions such as location tracking/analytics, band steering, roaming, wireless signal and power management etc etc. 

RansNet HSG can also act as a high performance router (to directly connect to ISP ONT), therefore eliminating cost of another Internet router and firewall. If there's already bundled ISP router in place, the HSG also doubles as a perimeter defense stateful firewall, in addition to any other layer-7/UTM firewall that STC may have.