My current car has an automatic transmission, and not only have I heard it performing engine braking on its own while in cruise control, but I was successfully able to perform it using the manual override this morning to keep my speeds down and compensate for the loss of braking power.
Automatic transmissions use a torque converter -- oversimplified imagine two propellers in a liquid, one powered, & the other connected to the transmission. A locking torque converter [which most vehicles have nowadays] tries to do just that, lock at higher speeds. A manual transmission uses a clutch, where a disc with a material similar to brake lining on both sides is squeezed by spring tension between the engine's flywheel & the transmission's input shaft. When you step on the clutch pedal you release that spring tension -- the more you press down the more spring tension is reduced.
So... you should be able to see or imagine why a torque converter might be less ideal for downshifting &/or braking. Will it work? Yes, but it's better suited to limiting the gear to a lower range, limiting speed going downhill in the mountains.
And this is one place that automation can improve considerably. On a vehicle where the PCM is able to recognize when engine braking is about to take place, the torque converter can be instructed to lock up and stay locked once the transmission is in a suitable gear. I suspect my car is able to do this, since I've seen it engine braking all by itself while in cruise control and going down a steep hill.
Like so, engine braking with an automatic transmission is as practical as it is on a manual transmission, because the only difference between the two is the slightly higher slip factor resulting from the design.
Designs that don't do that though, you are indeed correct that engine braking with an automatic is less effective than on the same vehicle equipped with a manual transmission. Usually you end up going a gear or two lower into the range of options with an automatic in order to bring the RPM up enough to make the torque converter couple it back to the engine.
And as for engine-braking, I recall him saying that it was to be avoided at all costs, as, not only could it make for really jerky driving and risk hurting passengers' necks, but also, with constant use, it would destroy the transmission as you were making it do something it was not designed to do (it was designed to transfer the engine's energy to push the car forwards, not slow it down).
May sound logical to someone without mechanical knowledge, but say that to a decent mechanic to make him/her laugh. Did he know how unnecessarily hard it would be to manufacture gears that had only one face of the teeth hardened?
There actually is a mechanical concern to engine braking an automatic now that I think about it. It has to do with the way automatic transmissions shift- using bands similar to brake pads to grab different areas of the planar assembly in order to change the gear ratio without ever actually meshing or unmeshing the gears.
If you apply too much torque to this assembly, it can cause the bands to slip. This results in increased transmission wear and heating, and could indeed ruin the transmission.
However, this only applies to automatic transmissions, and really is only a concern in a scenario where the transmission is already worn out to a point where it is already experiencing internal slippage problems. A transmission that still has plenty of band-meat left and is operating at or above its optimum shifting pressure will have sufficient resistance to internal slip that engine braking won't hurt it any more than smoking the tires at the redlight- because again the tires will lose traction long before anything bad happens mechanically inside the transmission.
Right from the start it needed new brakes, and one of the brake lines got changed at that time because I had to cut the line to get the caliper hose off. But for the most part the only time I even look at the underside of it is when I think there is something odd happening, and having the brake lines rot out and pop like this happens every couple years.
What you need to consider is total age & condition plus the possibility of contaminants. If one PC case fan goes out, myself I'll replace all the others that are the same age rather than assume it was a fluke. If one rubber brake line rotted, assume all are the same age & replace them. BTW, heat + penetrating oil + a crows foot type fitting socket connected to a small impact help loosen rusted fittings. The impact can be key because of the rapid impacts -- not raw horsepower.
I mentioned contaminants... ANYTHING but pure brake fluid [of the correct type for your car/truck] can trigger chemical reactions that will swell &/or eat rubber seals & lines. THIS IS IMPORTANT. Rubber lines rarely go bad -- I can't remember replacing one in years running dealer service. It could be a combination of age & environment, e.g. in the Detroit area winter road salt wrecks havoc, but if you're repeatedly having problems with rubber lines consider the possibility that water or something else has gotten into the brake fluid, &/or that the wrong type of fluid has been added. I have seen where that led to ONE of the brakes locking on -- at high speed! Imagine what happens when a wheel stops turning!
I've never seen rubber brake lines. Its always steel, sometimes copper or plastic coated in a futile attempt to stop them from rotting out so fast. If only I could get stainless steel brake lines without breaking the bank, they'd stay pristine for the entire lifetime of the car.
Usually whenever I have to work on the brakes at all, I'll bleed the system down to nothing and refill it with fresh fluid from a sealed container. Contaminants eat calipers and master cylinders, and even if you keep the system tight they still manage to get in there. Fluid is cheap, and if it keeps me from having to change anything more expensive than the lines I don't mind paying that much.
Money is tight for me though. Simply replacing things whenever I happen to think about it would leave me completely broke. Though I prefer to not have it break in the first place, when you're limited on cashflow its best to stick to if it ain't broke don't fix it. And then make a point of learning how to ride out any failures that do happen, cause it will break no matter what you do it.