I've heard that hand sanitizers may not be as effective against certain types of protein-based bacteria, or something along those lines, but beyond that I can't seem to find the line where hand sanitizer alone is longer enough.
This question seems to ask in which situations hand sanitizers can be a suitable substitute for water and soap.
Soap and water is still considered the gold standard for hand hygiene.
When using soap and water, it is important to wet the hands first. Then apply 3 to 5 mL of soap to the hands, avoiding bar soap. Next rub the hands together for a minimum of 15 seconds covering all surfaces of the hands and fingers. Finally rinse the hands off with water, dry thoroughly with a paper towel and use the paper towel to turn off the faucet.
When soap and water is not available, alcohol-based hand rubs (wipes, gels, or foams) should be applied. When using an alcohol-based product, healthcare workers must completely follow the manufacturer's label to ensure that the desired efficacy is reached.
To answer the initial question properly we have to differentiate the situations:
- daily setting, "normal world": almost never; only when detergents and water are unavailable and an actual incidence would advise disinfection
- health care setting, medical world: still almost never, as a substitute, but as an important addition to water and detergent
For a General Question of daily Hygiene:
The main effect of a handwash is that it washes away the bacteria/germs. Water, especially warm and hot water does its own thing to remove them, a soap-like substance adds to this effect and so is a towel. Any added ingredients that qualify for actually killing germs are just icing on the cake. None of the above methods I listed will so much kill but remove bacteria or dilute them in the sense of reducing their numbers. Nothing that is not also harmful to you will kill all of the bacteria. As long as you are not ill and required to be or live as sterile as possible that is a good thing.
You are yourself a living being. Sounds like a fun fact but is meant to convey that "a human" might have different definitions now than a few decades ago. Not all bacteria are harmful. Most of them are not. Many of them are actually beneficial or even needed, like those in your gut. While gut bacteria might have quite a good reputation by now, those on your skin, that those handwashes would like to kill are only slowly getting a better stand. Everyone lives in a symbiosis with their individual microbiome. Overzealous sanitation at least disrupts this balance. Costs and benefit should be carefully calculated.
Normal handwash is usually (more than) enough. It is not possible to really sterilise everything completely, nor would that be desirable.
Add to that the effects of evolution: What does that mean in the slightly longer term? In using sanitizers regularly you create an environment on your skin where the pressure to adapt is directed towards resisting the chemicals used in that agent. Some bacteria survive this attack. Compare that to their phenomenal ability to multiply and overcome chemical onslaughts with for example antibiotics. This leads to the situation that you un-train your own immune system to deal with any bacteria and disrupt the workings of the good bacteria your microbiome needs or can tolerate. Indiscriminate killing also tends to have the very unwelcome side effect of giving just the most harmful bacteria an edge in the fight for survival.
Hand sanitizers have bactericidal and virucidal properties that when used properly in sufficient quantity for at least 20 seconds kill the vast majority of pathogenic organisms/viruses. Alcohol-based sanitizers are superior in spectrum of what it kills.
However, you are correct that it doesn't kill everything, and some of what it doesn't kill is pretty nasty stuff.
For example, spores are formed by some bacteria, including Clostridium difficile, (aka C diff) which is a hard-to-kill bacteria that can cause severe diarrhea (usually in people whose immune system or gut flora are disrupted). It takes substances like bleach to kill the spores, so you have to wash them off instead.
Similarly, fungus isn't always killed by sanitizers.
From the CDC's "Show Me the Science - When & How to Use Hand Sanitizer"-
Washing hands with soap and water is the best way to reduce the number of microbes on them in most situations. If soap and water are not available, use an alcohol-based hand sanitizer that contains at least 60% alcohol.
Alcohol-based hand sanitizers can quickly reduce the number of microbes on hands in some situations, but sanitizers do not eliminate all types of germs. Why? Although alcohol-based hand sanitizers can inactivate many types of microbes very effectively when used correctly, people may not use a large enough volume of the sanitizers or may wipe it off before it has dried. Furthermore, soap and water are more effective than hand sanitizers at removing or inactivating certain kinds of germs, like Cryptosporidium, norovirus, and Clostridium difficile.
There's also a short novel on this topic.