You can get pretty far if you don't have much in the way of concurrent writes. Concurrent writes can be safe mind you, but that safety comes from locks that are polled against at increasingly long intervals (at least in my experience backing Django with SQLite). If you have one thread making many short writes over a long period, another thread will check to see if it can acquire a lock at 1 second, 2 seconds, 5 seconds, etc and if it doesn't time that check exactly perfectly you'll keep polling until your timeout. That is, out of the box you don't have a fair queue for the write lock. That kind of access pattern doesn't describe all apps of course! And in WAL mode your reads won't be blocked by these long sequences of writes.
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