You need to be specific about the make/model and even the firmware version of software you're talking about before you're going to get a real sense of what the risks might be of losing a camera from a reboot. It depends almost completely on the manufacturer and software. With the network environment being a wild card.
As a manufacturer, I feel like even very very low failure rates of our cameras is unacceptable. We happen to monitor every one of our cameras deployed, and try to do a root cause analysis on any failures.
Most manufucturers use techniques common in embedded systems to ensure availability, including:
* Use a 'factory default' OS image to fall back to in the event the current firmware image fails to boot a certain number of times in a row--this is designed so a 'bad' firmware update will not brick the device.
* Use a hardware watchdog that must be 'fed' periodically by higher level software functions or the camera will reboot--this technique tends to work around deadlock or other failure conditions of the higher level software.
* Use of image based (rather than incremental) updates--designed to better manage the final configuration of the software and prevent bugs in the update process itself from inadvertently causing the software to diverge into an untested configuration.
Even the most reliable IP devices do have a hurdle to jump when they reboot though, and that's rejoining the network. It can occasionally be like walking outside to get the newspaper and finding you accidently locked the door behind you. This depends on the network and the camera's setup. I just point it out because it's an area of risk where even the most reliable cameras may stumble.
My advice is if you 'feel' like you're at risk of losing cameras after a reboot, then you should find a new vendor. The manufacture's job is to make you feel like their equipment is reliable and trustworthy. If they can't do that then somebody else can.