They have a ceiling effect that is theorized to occur because of how they work on the GABA receptor (the receptor that is responsible for its mechanism of action). They are thought to work similarly to many other anesthetics such as propofol however their effect has a dose ceiling. I disagree that it is hard to die from OD, if you have the right comorbidities or are using them along with other drugs such as alcohol it is very easy to OD.