That’s a bit of a loaded question. The v1, v2, and v3 SmartThings hubs are dependent on the cloud for most things, so when your internet is down, or the SmartThings cloud is down, so is your security system. These weren’t really designed with security in mind, and are therefore lacking in several regards when compared to systems designed specifically for home security.
The ADT SmartThings hub was designed with home security in mind, so it has cellular back-up, can be armed or disarmed locally (not cloud dependent), has entry/exit delays, has an officially supported keypad, and comes with a touchscreen with a built in siren. One downside is that only ADT sensors can be used for the security monitoring with the ADT hub. They’re clunkier and quite a bit more expensive than many of the other sensors available.
Here’s a thread discussing the differences between the standard hubs and the ADT hub in more detail that you might find worth a read:
https://community.smartthings.com/t/if-not-using-adt-monthly-service-is-it-worth-it-to-get-components-self-monitoring/122880
Yes, there are several external sirens that work with SmartThings. Zooz, Dome, FortrezZ, Aeotec, GoControl, Zipato and others all make sirens that are compatible.
Yes, your Arlo cameras, Nest t-stat and smoke detectors, and hue bulbs can all be linked to and controlled by SmartThings.