Devices
While FlashFire has been tested on many different devices by both developers and users, of course not all device brands and models have been extensively tested. It may be worth visiting XDA-Developers.com and getting advice on FlashFire usage in your specific device's subforum.
Google devices are the main development platform, and thus generally well supported.
Samsung
Samsung devices are generally well supported, but sometimes their quirkyness needs to be worked around. If you run the app on a Samsung device, a card will be present on the actions page which will provide you with detailed usage instructions when tapped. Failure to follow those instructions may lead to (soft-)bricks.