If you are going to self-monitor, are you going to have your phone with you 24/7, always charged and never in silent mode? In the shower, while sleeping, etc.? You're never going to be in a place with poor or no cell signal? For most people that's impractical.
Are you willing to take the chance, however small, that the alarm will go off and you didn't hear the notification on your phone and missed the fact that someone broke in, or worse, that a smoke detector went off? Or the internet and/or power at your vacation house goes down?
Home automation systems and security systems are two completely different animals. It's one thing if a light doesn't go on or off at a scheduled time; it's an entirely different matter if a security or safety event occurs and nobody knows about it. Home automation systems are simply not designed for that level of reliability.
If you're really concerned about costs, look at the Ring Alarm. At least it's UL rated and meets certain criteria for reliability and robustness and I think it's only $10/month. Personally, I would not trust the safety of my family and property to anything other than a professional-grade alarm system that uses cellular communications. You don't have to spend a fortune for one, you can install it yourself and I'm only paying $25/month for professional monitoring services including real-time two way voice communication with the monitoring center from my alarm panel.
There are various ways you can connect an alarm system with a home automation system, depending on the particular systems you choose.