The commercial offerings all seem to be based on ancient technology with hacked add-ons to provide wireless and internet connectivity etc.
In the case of my Bosch alarm it's cellular comms module periodically gets confused and the whole thing has to be power cycled to bring it back to life, then I have to manually reset the clock because it doesn't have a battery backup and it doesn't know how to get the time from the internet - unbelievable!
From tinkering all I can gather is that the sensor inputs do loop resistance measurements to detect tampering/open circuit, short circuit and alarm conditions. The alarm outputs also do loop resistance measurements to detect tampering.
Is that basically it? The logic doesn't seem particularly complicated, although the Bosch programming app makes it look complicated.