currently I am working on a pcb which is running with an Atmega328p. I wanted to make it Arduino compatible, so I burned the bootloader to the MCU. But after trying to upload sketches at 3.3V, I noticed, the controller was not recognized or couldn't communicate. There are some threads and blogs about problems with the brownout settings, but I think the extended fuse byte in boards.txt of "Breadboard Arduino with internal 8MHz" should be changed to 0x06 in order to avoid problems, wenn connecting 3.3V from external supplies. I know, this problem usually happens, because of the load of the whole circuit, anyway, to make programming more robust, this should be changed. I think there are a lot of people like me, which try over and over to find the problem of their circuit just because of this. Also I think this BOD setting makes more sense, considering lots of those Arduino compatible made pcbs are running on batteries. The MCU can work stable down to 2V.