Per ounce, they have the same amount of caffeine, sometimes less, than the comparable cup of coffee. It's those other stimulants that get you in trouble.
I've tried a number of energy drinks in my lifetime. Most are OK in small amounts, but watch out if you see L-carnitine. Most drinks don't use it but Monster Energy does, and it has serious side effects. It altered my mood (made me inexplicably angry) and wrought havoc on my heartbeat and breathing.
That's not to say that the other stimulants don't have an effect. They do, so you don't need as much to get the same buzz. Plus, if you're watching your sugar intake, that's something else to beware.
In short, that 8 ounce Red Bull probably won't be a threat to anything and may even be beneficial. That 24 ounce Mega Monster, on the other hand, might kill you.