Seems to me that for a personal tool like this, sqlite3 is non-problematic.

https://www.sqlite.org/whentouse.html

"Generally speaking, any site that gets fewer than 100K hits/day should work fine with SQLite. The 100K hits/day figure is a conservative estimate, not a hard upper bound. SQLite has been demonstrated to work with 10 times that amount of traffic."

Any concrete reasons? SQLite is probably good enough for 99% of websites / apps.

It's not designed to be straight exposed as a web service.

It's not hardened to handled malicious traffic.