Although I think your worry about quality excessive, it's not like making pills is rocket science. The supplement companies most likely get the powder in bulk from biochemical factories that also manufacture prescription drugs.
The concern is not excessive, at least in the US. Consider https://consumerist.com/2016/07/27/anyone-can-make-market-a-... :
> While real drugs — prescription or over-the-counter — must demonstrate levels of safety and effectiveness before hitting store shelves, dietary supplements are effectively regulated like food products.
https://consumerist.com/2016/01/20/7-things-you-need-to-know... gives a summary of a Frontline episode on the topic; Frontline does documentaries on public affairs topics.
Or, from 2001, http://ajcn.nutrition.org/content/73/6/1101.full - "All plant products were correctly identified by botanical plant species (ie, Panax species or E. senticosus); however, concentrations of marker compounds differed significantly from labeled amounts. There was also significant product-to-product variability: concentrations of ginsenosides varied by 15- and 36-fold in capsules and liquids, respectively, and concentrations of eleutherosides varied by 43- and 200-fold in capsules and liquids, respectively."
Or from the 1990s, http://www.sportsci.org/jour/0003/lmb.html :
> Analysis of 16 commercial DHEA products revealed that only half the products contained the amount of DHEA stated on the product label; content varied from 0 to 150% of the stated content (Parasrampuria et al., 1998).
> Melatonin supplements have failed to meet quality claims or delivery profiles stated on their labels (Hahm et al., 1999).
It lists others as well.