There is a light on the dash as well that tells you the hand brake is on, just the same with a manual hand brakePretty sure that are. You listed benefits. Here are a few problems:
They are less reliable. That's not a guess. Talk to a mechanic, they don't speak very highly of them. One said to me 'They feck up all the time'.
When they do go wrong, they are harder and more expensive to fix. If you want to fix it yourself you're dealing with a more complex caliper which you can't manually wind back. It's an electro-mechanical system rather than a fairly simple mechanical one.
But the real problems are user interface.
A manual handbrake can be operated without looking at it with tactile conformation. You know if the brake is off, on and even how hard the brake is on by feel. The switch of an electric brake gives no feedback and required an LED tell-tale to let you confirm that the brake is activated, which you have to look down to see. Anything which takes your eyes inside the car unnecessarily is not an upgrade.
Electric brakes are slower in operation. You can drop a manual handbrake off instantly at exactly the time of your choosing. Electric ones are much slower as the motors take time to wind back making it difficult to time the transfer of power, when picking away up hill for example. Less of a problem in an automatic but don't get me started on them!
Also, you never have full control over the brake. What is actually happening is that you ask the computer if it could please let the brake off, and it might disagree with you. Depending on the car, it may not release the brake if a door is open for instance. In practice, this being second guessed by the car leads to confusion and lack of confidence that it will do what you want when you want it done. Or conversely, overconfidence that the car will make the right choice.
You have no control over how hard the brake is applied. To cover all eventualities, an electric brake has to put the brake on very hard every time. Most of the time you don't need the brake on that hard and only really crank the brake on when parked on steep hills. Putting it on really hard all the time puts more strain on the parts and makes the brakes more likely to freeze on it winter.
Less feedback in a fault or misuse situation. If a manual handbrake fails, you feel it. You know the brake is not working correctly. An electric brake leads you to trust the car with control of the brake so if there is a fault you are less likely to pick up on it. With the car directly controlling the brake instead of you there is more chance of a disconnect between what you expect and what has actually happened, which the OP knows all about!
For a real driver certain things lead to satisfaction in a car. The feeling that you are in control of the car, the car is effectively communicating what it is doing and the performance of the car closely matches your desires and expectations. Electric handbrakes are compromised on all three. Which is why electric handbrakes are crap!
Also as others have said you can feel the handbrake if your foot is on the brake if you need to apply it. But then realistically you should leave it in gear on an incline anyway.
A handbrake should only ever be applied right enough to stop it rolling. Loads of people seem to feel the need to apply it as hard as possible stretching the cable etc.
Anyway I don’t disagree with your points on what can go wrong. However, that doesn’t make it a terrible idea, that assessment is what I disagree with.
There are plenty of things out there that you could call terrible simply based on what happens when they wrong, yet we still enjoy the benefits of them.
If we always focused on what could go wrong with things then life would be pretty dull.