Once entered a STARTUP2 server cannot be stopped correctly, only killed via `kill -9 <pid>`. It probably will stop gracefully once through it's STARTUP2 phase and into RECOVERY or SECONDARY, but STARTUP2 may be quite a lengthy process (days) if large databases are to be replicated.
Also once restarted after unclean shutdown replica Secondary wipes out all halfreplicated files and starts over again from scratch.
This makes replication essentially one gigantic atomic operation, exteremely lengthy, unmanageable and unreliable.
This behaviour should be either documented or (ideally) fixed, making initial replication interuptable process.