If you breath through a vacuum cleaner hose that pretty well captures what the sensation is like. It is not as reassuring as open circuit (OC) in that with OC there is that big differential in pressure, that gas "wants" to get into your lungs and if the regulator fails it typically fails in a failsafe manner i.e. too much gas rather than not enough.
RBs have 3 main failure modes that are pernicious.
Hypoxia, No warning as the point where you might think that something is weird is right about the point where you are powerless to do anything about it.
This is a very good example of being powerless to effect your own survival.
Hyperoxia, No warning (some claim facial ticks and visual disturbances but this is greatly variable) you just start convulsing and drown due to the mouthpiece coming out of your mouth.
Hypercapnia, little warning and only remedied by going to an OC source of gas and getting your CO2 levels down. If no action is taken then death is the only outcome. One of the truly nasty things is that CO2 increases breathing rate which lowers the efficiency of your breathing which increases CO2 which increases narcosis which decreases cognitive function which <insert accident here>.
There are protocols that mitigate the risks of these occurring somewhat and establishing the correct monitoring habits is an essential part of learning.
I am using a full face mask to reduce the risk of drowning should I have a incapacitating event underwater.
The proper understanding of what can go wrong and how coupled with good pre dive checks and post dive cleaning and maintenance is what keeps you alive.
There is a lot of debate over whether there is an inherently safer RB design. You can have a manual or an electronic/automatic design. The manual design has an orifice that allows the constant injection of O2 set to your metabolic rate. You are then responsible for the manual addition of O2. Some argue this engages the "I don't want to die" circuits in your brain and ensures you monitor the PPO2 in an attentive manner.
The Auto proponents like to leave the maintaining of PPO2 up to a computer and still try and keep an eye on the PPO2 as well.
The lower death rate on manual RBs seems to suggest a safer principle but the statistics are skewed a little by the reluctance of manufacturers to be entirely candid about production numbers.
I dive a manual unit. It means there can be a bit more task loading at certain points of the dive but it's ok.