Selling point I did not expect: "our pageviews never leave your server" closes deals against the big platforms. Privacy is a feature with a price tag. 300 instances running, most on a 4-euro VPS next to the app.
Technical notes: columnar storage for events (Parquet + DuckDB for queries, Postgres for metadata), aggressive 1-minute rollups, raw events deleted after 30 days unless you pay for retention. The dashboard is one page and loads in 300ms because there is exactly one page.
Hardest part: bots. A third of "traffic" is junk and every customer discovers this during the trial. Built-in bot filtering is now the demo moment, not the charts.