It probably is possible to hard brick using dev flash mods somehow, if you really tried, but anything you might do by mistake, just deleting files wouldn't do it.
Not recommending anyone do this, but I have tried deleting all of dev_flash contents and rebooting before and it it just asks you for firmware PUP. CoreOS that has safe mode and other critical files are not stored in dev_flash. You can actually delete everything on dev_hdd0 too and it just acts like you did factory reset really.
There are probably other ways, But the only way that I know of that you can hard brick with only changing something on dev_flash is on older Cobra CFW that had no failsafe, if you put the wrong Cobra payload version onto a CFW NAND console there was no way to disable it and it would hang at boot and not be fixable.
