I've slowly evolved from just writing to and looking up json files to using SQLite, since I had to do a bit more advanced querying. I'm glad I did. But the defaults did surprise me! I'm using it with php, and I noticed some inserts were failing. Turns out there's no tolerance for concurrent writes, and there's no global config that can be changed. Rertry/timeout has to be configured per connection.

I'm still not sure if I'm missing something, since this felt like a really nasty surprise, since it's basically unusable by default! Or is this php's PDO's fault?

This is the fault/price of backwards compatibility. Most users of SQLite should just fire off a few pragmas on each connection:

    PRAGMA journal_mode = WAL
    PRAGMA foreign_keys = ON
    # Something non-null
    PRAGMA busy_timeout = 1000
    # This is fine for most applications, but see the manual
    PRAGMA synchronous = NORMAL
    # If you use it as a file format
    PRAGMA trusted_schema = OFF

Also use strict tables. https://www.sqlite.org/stricttables.html