Metformin is one of the most often prescribed drugs, particularly in the older population that might also suffer the most severe complications of covid19. If this is indeed fatal in humans you as a doctor have to be quite sure that you have the correct information on the patient before you attempt to treat with this.
I will also severely limit the scope of people who can be treated this way.
There is a lot of work done right now on formulation of defined faeces microbe sets, partly because most people expect that ultimately these treatments will have to be approved by the authorities. This is probably a good idea, as the microbiome consists of both bacteria, virus and eukaryote organisms, and the survey people use for 95% of all microbiome work can only capture the bacterial fraction, so a faeces transplant that look good from the bacterial perspective could potentially include nasty virus or parasites. How long this work will take is difficult to say, but years at the very least.
The second part is that right now it is somewhat unclear what is actually microbiome related problems. Clearly, associations to all kinds of diseases and conditions have been made, but the casuality and dynamics of the interaction with the immune system and host factors is not well-defined for now.
It seems obvious that diseases in which we can intervene with a fecal transplant and improve an outcome are microbiome-related, but there isn't actually that many solid cases for this. C.diff infection is well-established. A recent RCT looks really promising for insulin resistance (i saw the senior author talk about it at a conference and i was certainly convinced), but naggingly insulin resistance wasn't actually the registrered primary outcome of the trial
It isn't really clear that synbitoics work yet, the evidence for metabolic outcomes at least is sparse as of now
http://www.mdpi.com/2073-4425/9/3/167/htm (see figure 6)
I work in the microbiome field, and colleagues and i discussed this paper a few days ago. The overall impression seems to be that this is not for the practically-minded but primarily science for scientists. A lot of microbiome studies have suffered from inadequate exclusion critera because of the lack of this systematic knowledge.
I will also severely limit the scope of people who can be treated this way.