The most common problem is that when you do particle update to 0.7.0, the update is done in DFU mode (blinking yellow). The bootloader cannot be updated in DFU mode.
In versions prior to 0.7.0, the bootloader upgrade was inserted into one of the system parts. The problem is that in 0.7.0, on Photon and P1, there wasn’t enough space to do that.
The updating of the bootloader should be done using safe mode healer (blinking magenta) OTA if needed. However, if you get into a case where this is impossible, such as a device that does not have Wi-Fi configured yet, that’s where you most commonly run into problems. If you’re upgrading blank devices, you’ll probably have to manually upgrade the bootloader in --serial mode.
There might be other cases where you can get into trouble, but that’s the most common.