Out of curiosity, is this take informed by relatively recent personal experience? I experimented with storing a bunch of data locally using OPFS with DuckDB/Wasm, and it seemed to work fine in Safari and Chrome.

It's more aggregated experience over time. These issues often don't surface in the short term but instead show up over time with bugs in implementations in particular browser versions and whatnot. In some cases data just gets wiped and in others it gets corrupted, and with no server component and any backups stored in the same medium as primary storage you're left without recourse.

i built a "local" webapp once where the save button produced a file to download, in addition to keeping things in local storage. then if your localstorage got wiped, you changed browsers or whatever, you could load that savefile to restore your data.