Thanks for the reply. Yes, the server going down would stop the synchronization but I would think it would optimize the synchronization too. Instead of each client needing to synchronize to 5 devices, it can simply upload to the server and then it could push to all the other devices.
This may be the case, e.g. when some of the devices are located on very slow networks, etc., but if we’re talking about the same LAN (or fast connections in general), then it should be faster to download a file from 4 other devices simultaneously than having everything go through the single server. Syncthing splits data into blocks, so it can download the same file from multiple sources at the same time.