Anything with a write-based working set that fits in the buffers of the primary instance;

Databases that are unused most of the time;

Databases with a lot of read replicas (potentially with Neon only providing the read replicas, not the write node/hot standby);

Apps that want to run analytics on the data, but don't want to transfer O(datasize) data to a different DB every time, and also don't want to deal with the problems of long-running transactions.

Not very suitable:

OLTP with writeset that doesn't fit in the caches;

Databases that need <1ms commit latency.