Search the forum for security discussions and I think you may find that some of the features that you would expect a security system to have are absent from SmartThings.
Just as an example, there are no entrance or exit delays; if the Internet is not available the system cannot send notifications and you can neither arm nor disarm it; there’s no concept of a master user so anyone you give access to, say, turn lights on and off can also change anything else on the system; and the company’s own official product usage guidelines tell you not to use it for anything critical.
https://www.smartthings.com/guidelines
do not use SmartThings to control any device that may have an impact on health, safety, security, property, or financial interests,
And they can and do push out updates at any time which can take your system off-line temporarily, and there’s no way to delay or refuse those updates. We usually get a few days notice ahead of time, but not always.
It can still be very useful as a system for convenience notifications (at our house we use it to get a notification if the guestroom window is left open and rain is expected), but just understand that it doesn’t really go head to head with a purpose built security system, even a relatively inexpensive one. So just know what you’re getting.
As for Arlo, there’s an official integration. Hopefully other community members who have the Arlo cameras will be able to say more. 
https://support.smartthings.com/hc/en-us/articles/209529506-How-to-connect-Netgear-Arlo-Cameras