I have a remote pi 3B+ doing important home automation tasks. After having some SD cards broken due to power failures, I have prepared the set up with an SSD. The pi boots correctly without sd card, however booting proved to be unstable. Sometimes the pi does not reboot after a power outage. This failures are very short in time, it happens that other devices are not restarting at all. I think, this may cause a problem for the pi usb boot. As a quick solution, I have placed an other pi 2 next to it with a relay. This one boots every time without any problem after power failure from sd card and I use this one to restart the "main" pi 3. When I pull down the power for 10s for the main pi 3, it always reboots from the usb.
I will definitely install a ups, but this wil kind of masking the real problem.
How could I really improve booting stability from usb?