Guys, these tests are mostly a waste of money right now. See the "testing" section here:
https://www.reddit.com/r/HumanMicrobiome/wiki/index
I agree the tests have limitations, including providing information that we don't yet know how to interpret, but I most definitely disagree that they are a waste of money.
They can be very helpful in giving an overall picture of what is going on in the gut to counter the extremely distorted picture presented by culture based tests (CDSAs) which try to convince people they have overgrowths of minor aerobic species.
They can show if there are serious imbalances in potentially harmful genera that may warrant treatment.
They can show the extent (or lack thereof) of diversity, something that has been shown consistently to be associated with adverse health outcomes, flagging that serious efforts need to be made to adequately feed the gut microbiota.
For those of us who like to have as much information as possible, following the gut over time can help us to see if our efforts at gut improvement are bearing fruit.
I was not particularly impressed with the selective use of information in the link you provided. I strongly take issue with your microbiological "expert", who seems to be using classic smear techniques to make a case for a predetermined conclusion, rather than attempting to lay out the facts.
16S sequencing of stool samples is the mainstay of all microbiome research, used and validated in many thousands of research papers, it is not some clever way that a few companies have devised to fleece us of our money by performing useless tests.
Yes this technique does have limitations but it also has many pluses, including the capacity for extensive sampling of any microbial environment quickly and cheaply. This is why it is the workhorse of microbiome research.
Shotgun metagenomic sequencing does give much finer detail but it is much more time-consuming and expensive. It is essentially a research tool which might be used to answer questions raised by earlier 16S studies.
Yes stool samples are studied because they are easy to obtain but results have been compared with those obtained from biopsies throughout the gut and we do know quite a lot about the differences and similarites. Those many thousands of research studies have concluded that stool sampling is a valid way to sample the gut.
To answer particular questions that require detailed knowledge of the gut mucosal surface at a particular location, yes biopsy would be required, but this invasive procedure is not necessary for routine sampling.