It’s good you’re inquiring because the technique described in this topic, although easy, is not without risk.
Exposing any device to the internet is a calculated risk. The connection is as secure as its weakest link.
The risk is what I explained in my previous post. You now have a device on your local network that is always accessible to the internet via port 8123.
The only thing that prevents unauthorized access is Home Assistant’s authentication system and the strength of your chosen password … and the assumption that there are no existing security vulnerabilities (like the one discovered in 2023).
If you have other questions like how can your IP address be easily discovered then you should familiar yourself with automated port scanning and how it is commonly used to discover potentially vulnerable assets.
FWIW, I use Tailscale to access my Home Assistant server but via its VPN and not its Funnel feature; the server is not continuously exposed to the internet via port 8123. That’s my “comfort level”, especially after CVE-2023-27482.
EDIT
I’ll go out on a limb and say that this technique is probably no less secure than what you get with a Nabu Casa subscription.