Which sample was ever found with cyanide?
I think it's easier to talk specific examples instead of non-existent or unlikely hypothetical examples. No test in the world tests for everything
That's sort of like going to the hospital because you feel sick, and asking for them to blood test for every existing and extinct disease in the world. They won't do that, even the most professional hospital. They will first look at symptoms, at prevalence of different diseases , etc.
Sometimes a simple test may be enough for what you are looking for (like a pregnancy test), which will already reduce the probabilities... but of course maybe you think you are pregnant but you actually a weird new uncatalogued disease with very similar symptoms. And maybe the pregnancy test doesn't differentiate the disease. But that would be a rare case and wouldn't mean pregnancy tests are useless. They are useful for their particular application, for a large number of people, if you are aware of limitations..
It's the same with drugs. When harm reduction work is done, what you need to do is reduce the most dangerous possibilities which you know can happen. So you have to look at what substances are most commonly used as adulterants, and this can help you avoid some main dangers. You cannot test for everything simply because that's impossible. You could conceive of any number of synthetic substances that shows exactly the same as a wanted substance in even the most expensive tests, and yet are way more dangerous. But if you look at number of deaths and hospitalizations, you generally see a handful of substances which are causing the problems and that we can avoid by doing some simple cheap tests.
I have access to international testing statistics, and other people can watch out for harm reduction groups' twitter feeds and results from ecstasydata.org to see most common adulterants and be aware of trends in adulteration (for example there are some superman shaped pills going around which have high levels of PMMA, please be careful people! )
Say for example LSD. LSD is either LSD, or an NBOME related substance, or a DO(x) substance, or bromodragonfly (nearly inexistant these days). With simple ehrlich reagent, all of these can be eliminated. That is incredibly useful. Sure they don't test for a hypothetical unknown novel totally toxic ergot-based substance that reacts exactly the same, in that case you'd be screwed, but what are the odds, and what is the alternative you are offering (not taking at all, or taking it but not using any reagent) ?
With MDMA, things get a bit more complicated, since MDMA's reactions usually mask PMA/PMMA's reactions, and those are the more dangerous adulterants. So in that case yes it's important to recognize that limitation and not give people false sense of security of marquis turned black.
With synthetic cannabinoids, I'm not sure if by now we have indication on which ones are more or less dangerous. Some can be differentiated with different reagents, I can check out the list.
Again, TLC can def help.
As for cannabis, it depends a lot where you are from. There are specialized tests for fertilizers/pesticides/heavy metals, but generally what you will find is more regarding THC/CBD ratios. Or also terpenes, some people might be interested in the info but it's not very useful so you'd usually have to get some specialized testing for that. It's all possible though.
In the few places where cannabis may be sold laced with synthetic cannabinoids (if this ever happens), I think TLC or sending to the people I linked before would be the best bet.