Can I still use frozen beef brisket left out at room temperature for 12 hours?
Unfortunately, as sad as it is to see two nice briskets get wasted, I would not feel comfortable using them. In the guidelines from the USDA, thawing on the counter is explicitly discouraged. Additionally, it is likely that in that amount of time the surface of the cuts came up to temperature to allow for otherwise inactive (due to freezing) pathogens to become active again. In the absence of a vacuum pack and having been left at room temperature for that long, it is likely that even in the original paper wrapping the food has become unsafe to eat.