If a device hasn’t been rebooted in a long time there is a much higher chance of it not coming back after a reboot. This is made worse by the fact that sometimes power loss is unexpected which means that an outage can occur at a bad time.

The other issue is that a high uptime device doesn’t usually have the latest updates installed. Delaying updates creates security issues and when you do get around to updating it means that more things get changed at once.