Submarines are complex devices. Nuclear submarines are even more complex. heyoka is correct in noting that they do break. But I hardly think the nuclear or conventional variety unreliable.

What makes submarines special is their ability to submerge. In order to accomplish this they have a pressure hull inside which no water is admitted, an outer hull which defines the boat's hydrodynamic shape, and which contains the ships ballast tanks.

In order for a sub to submerge, all that is required is to let air out of the ballast tanks, and admit water. In a World War II Gato class sub, the tanks were open at the bottom. All a sub had to do to submerge was to vent the air above the water, letting in sea water. To surface they pump compressed air into the tank, forcing the water out of the bottom, lightening the ship.

All these mechanisms for submergence involve a large and complex amount of plumbing that must be inside the ship. Valves may be inside the pressure hull, and those hull penetrations are potential weak spots. Everything breaks, and the more complex it is the more likely it is to break. So the more stuff you pack inside the small hull, the more maintenance problems the crew will face.

Ships float because they maintain positive bouyancy. The air that is contained inside their hull displaces an area where the sea water that would be there is heavier than the ship. Surface ships maintain a large reserve of positive bouyancy. This permits them to absorb damage and remain afloat.

A submerging sub is at a negative bouyancy. It weighs more than the surrounding seawater. When at the chosen depth its bouyancy is neutral. Any leak, however small, upsets that balance. The deeper the sub, the more serious is any hull penetration. If a sub is at, or near, its crush depth sea pressure makes the smallest breach a crisis.

In order to gain the ability to submerge, submarines give up the ability to endure damage. Even a surfaced sub only retains a small bouyancy margin compared to surface ships. Subs can be nicked, light bulbs popped, etc, but no sub will survive anything like the exocet missile hit that damaged the USS Stark.

The submarine's environment magnifies the problems further. A surface ship is always on the surface, so the crew doesn't have to worry about breathing, unless they are fighting a shipboard fire. That danger is localized. A fire on a submarine can quickly consume all available air, which puts out the fire, but suffocates the crew. Because a surface ship is on the surface, its crew has more time to sort things out. They can easily examine the outer hull, while working outside the ship is impossible for a submerged submarine. Also submarines ride much lower in the water, making them more prone to take on water when surfaced.

Submarines are incredibly useful to any navy. But we need to remember that they are both complex and fragile compared to other warships. Submarine accidents are in no way confined to nuclear subs, who differ only in their powerplant. Danger is a part of any submarine's life. Really, I think they do very well when you consider the additional problems posed by submergence.

Subs do have one advantage. If they are operating properly, they can go deep and ride out storms. Bad weather tends to be confined to the surface, so the ride can be very smooth at 200 meters or so below.