My experience has been that some are beneficial
if your problem fits into their niche of ability. Bar's Leaks is the best at sealing cooling systems, for example, but it won't cure a blown head gasket. The 'drain the oil and drive away' stuff works, just about any oil will do the same - for about six miles (depending on load, wear, clearances, etc. - YMMV.)
There are some friction modifiers for transmissions that not only work, but are required additives (in separate bottles.) Antifreeze doesn't lose its ability to not freeze, but its additive package that provides anti-corrosion qualities does wear out (sacrificially.) If the coolant is clean a simple bottle of additive is all that's required.
An old partner of mine went to school on the Caterpillar plan and a great deal of his education was centered around lubrication. They did myriad tests of lube additives, including everyone's favorite 'Slippery 49 + 1". In a nutshell nothing of noteworthiness occured. They came to the conclusion that it's damn difficult to make a teflon based product (PTFE) stick to something that's already coated with oil.
Injector cleaners are something that can be avoided by doing a few things: Buy gas from brand-name, high-volume retailers. Don't run on the bottom of the tank, cars run better full than empty. Not because the car will be sucking from the bottom, it does that no matter what. Because all petroleum products are hygroscopic, in other words they attract moisture. The lower you allow your tank to get, the more humid air that is sitting on top of it. The humidity is eventually absorbed by the fuel and all kinds of rude things result.
The only additive for power steering that I can think of is leak sealer. Most leak sealers for hydraulic devices are trying to work by softening heat-hardened seals. Once in a great while they may work (remember the niche) but what happens to the old, decrepit pressure hose that the stuff is running through? Softening it up is probably the last thing you want to do.
Same principle applies for transmission restoring additives - soften up old hard seals and rings and restore working pressures. Some are friction additives to give worn out clutches and bands a little more 'bite'. Hopefully they won't allow you to really tear things up inside before (when all the friction material is gone and running metal-to-metal) it heads for the happy hunting grounds.
Best bet is to do the service the OEM recommends, at their 'severe' levels. Oil changes at 3000 miles is a must. Oils just can't maintain their level of quality beyond that point. Synthetics can go many, many more miles without breaking down, but the problem for them lies in contaminants. Combustion ash and acids, moisture, fuel comtinants, etc. cannot be filtered, so the only way to remove them is by changing the oil.
Synthetics are great, but way too expensive. If you want more protection than the base mineral oils (and the last I heard was that Kendall GT-1 was still the best, with the other name brands in a clump not too far behind) try a synthetic blend, such as Dura-Blend, a 50-50 mix. It's less expensive and another up-side is as a 50-50 mix it's not as likely to seep past older seals.
Sorry for the filibuster, I get carried away occasionally
Beav