One thing to be aware of is if the auto reboot will go off right as the alarm would.
I wanted to try out a shorter reboot timer that would go off while im sleeping, so I set it to 8 hours and put my alarm in the owner profile. When I put my phone down to sleep, I noticed my alarm was exactly 8 hours from ringing, only a few seconds off (eg. Reboot would go off in 8 hours and alarm was in 7:59:55). I assumed this would cause issues but I let it happen to be sure.
As expected, I didn't wake up to any alarm. The alarm was set to "gradually increase volume", which was mentioned earlier in this thread as a possible explanation for alarm failure, but I highly doubt that was the reason in this case.
The two ways to fix this are to make sure you put your phone down at least a couple minutes before or after the 8 hour mark from your alarm, or to shorten the reboot timer to 4 hours. The problem with putting your phone down a couple minutes after the 8 hour is that it won't reboot before you wake up. The 4 hour timer is a better solution but I think 4 hours is too short and would greatly increase the chance that it reboots during the day. I don't want to type the BFU unlock in public unless I had already planned for it.
It would be cool if it could be implemented where "if alarm was ringing as reboot occurred, continue ringing when boot". It's probably complicated to do that, though, and definitely out of scope for GrapheneOS.