It always amuses me this division between 'medical' and 'natural'. A lot of medical was natural when it started, like that bloke hammering out willow bark on a rock by the river. Licking up the juice for his toothache.
You are quite correct that many, if not most, current medical active pharmaceutical ingredients (API's) were or are slightly modified versions of naturally occurring compounds. The difference is in the level of proof. Maybe we should think of these as 'statistically proven to have the named effect' vs 'someone is guessing that this may have some use possibly'. To get registered a new product has had to be proven to have an effect that is at least as good at solving the medical problem as the currently used alternative. This is done at multiple independent sites and each has to have the same outcome. The product can not cause more harm (negative side-effects, reduced survival etc than what is currently being used or results from no treatment). Almost as important is that in order to sell a pharmaceutical the manufacturer has to monitor any and all adverse events associated with the therapeutic. If any occur the use will be restricted or the product pulled. None of that proof of positive effect or monitoring of side-effects is required for a nutraceutical. Likewise manufacturing quality control and liabilities are lacking.
As has been noted a registered product has a very tightly defined manufacturing process and controlled set of ingredients in defined amounts. In the absence of that information a product can't get registered/sold. This continues to be one of the unspoken issues around medical marijuana. Last time I examined it over more than 400 chemicals had been identified in pot smoke. Not only had most not been studied for biological effect (they don't know what they do, or they are known as toxins so have not been studied for beneficial effects) but more importantly the ratios changed significantly depending on light, temperature, humidity, and nutrients used in growing the plants (clonal plants . So how can anyone say All Pot is good. Yes, I have no doubt some of the compounds are good for condition A while others are better for condition B. Thats all good but if plant X has different ratios than plant Y then how can a consumer/a prescriber/a retailer know if what is being bought or used will help a particular patient situation. A regulator/physician who has signed on to the Hippocratic Oath (Do No Harm) is hamstrung because they just don't know what is actually being administered and if it will help or harm.
One other reason NZ has so many of these nutraceuticals and issues around them is that NZ is one of only 4 countries in the world that allow direct-to-consumer medical advertising (Hong Kong 1950's, NZ in 1981, US in 1985, Brazil in late 2000's). While nutraceuticals are not 'medical' products and can't make claims of effect the flow on here has been a huge rise in their advertising (purportedly to compete with mass-market advertising of registered medical items), their use and the whole naturopath industry. Generous ACC/MoH subsidies of non-conventional medical practices has obviously also helped.
By way of full disclosure I have spent 3 decades working in biomedical fields -originally as a scientist, later commercialising university inventions and then as a VC and now as a consultant to companies and investors. The majority of projects have been regulated/registered products but there have been some nutraceuticals as well.