Biggest cost then would be electric. Older PC, probably...70 watts. So about 600kWh/year. Maybe about between $60-$150 per year.

Much cheaper than any hosting I know and bandwidth costs are absorbed into your monthly bill.

The real risk would be hardware failure. Hopefully you'll have backups or a user base that won't care if the instance goes offline for quite a while.

There's also a risk of unexpected security vulnerabilities letting an attacker compromise your public facing machine to get into your home network if you don't have it physically firewalled off.

Personally, I'll just let someone else deal with all the hosting issues. I'd rather donate if they requested than deal with all of that indefinitely.

There are a lot of genuine reasons people don't self-host from home, including, but not limited to: a) chance of security issues from firewall misconfiguration, b) mediocre speeds and intermittent service for people who don't have symmetric fiber connections, c) chance of security issues from RCE/shell access/XSS, especially bad if the server is not not properly isolated from the rest of the LAN, and d) TOS violations leading to disruptions in service (it's against the TOS for FiOS, for example, to host web-facing servers). If you're confident in your home labbing skills, are security-conscious, have a good connection, and know you won't tick anyone off, you're probably fine. For the rest of us, paying for VPS hosting is a lot cheaper than a zero-day screwing over your entire network, or an innocent mistake out of inexperience opening up major holes in your router firewall. You obviously get much cheaper hosting, but you also inherit a lot more to worry about in the process.