Yes you can reuse your sensors for multiple purposes in SmartThings. They can serve as sensor to trigger automations and also security apps and anything else. As JD pointed out you can buy an alarm or siren, pair with SmartThings and then use your sensors with the built in SHM security module from SmartThings or you can use any custom apps such this one to create your own security setup and trigger the alarm for an intruder alert or do much more.
Any system is only as reliable as its weakest link. You in this case you’ll have to ensure that your hub and internet are on a UPS for it to be reasonably reliable.