Just for the record. I am from Kenya and I write for an organization based in the UK. This is for all those so called "native speakers" I don't get what makes you special that you feel you must bash third world countries to get jobs. I don't need to go to such desperate measures.
You're correct in that it's only the desperate "writers" and company reps who resort to attacking legitimate writers on these forums in an effort to compete by defaming those with whose work they can't really compete fairly. (Coincidentally, most of the "writers" and reps who operate that way are incompetent ESLs themselves.) You're incorrect when you suggest that all criticism of ESL writers by native speakers is unfounded or based on their being ESL speakers. Most of us legitimate American and British writers have no problem with ESL writers who honestly disclose their being ESL and leaving it to customers to decide how much that matters to them (if it does). I have referred work to several (openly) ESL writers who are totally legitimate, precisely because they are honest about it and qualified to write at the level they advertise.
As much as it is true that Nigerians, Senegalese, and most of the Western African countries do have some unscrupulous people, it is a bad judgement call to act righteous about the fact that you claim native then you are better writers. So this is what I am going to do. I am asking any student to try me. When I write your essay or research paper well, you will come back on this site and tell all those people whose only credit is to bad mouth us, what kind of work has been done.
You're mixing two separate issues here: writing quality and legitimate writers vs. scammers:
On the issue of writing quality, I admit to never having bothered to identify country-specific English-writing patterns and idiosyncrasies, but I have been approached by many African writers asking for help finding work or offering to "help me out" anytime I have overflow emergencies. If they sounded legitimate and their emails were well written, I asked for a writing sample to review and in every single case, their writing was insufficient in quality to trust them with any work. There were stylistic consistencies, such as (1) paragraphs that consisted of nothing but repeating the exact same idea several times in slightly different words; (2) paragraphs that stated a premise, provided nothing to prove that premise, and then simply restated the same premise as a concluding sentence to that paragraph; and (3) unnecessarily complex sentence structure and vocabulary clearly designed to demonstrate what the writer hoped was evidence of language fluency in sentences that ultimately said nothing coherent whatsoever.
The same was true of one African writer who had previously asked me for work and subsequently accepted a project (independently) from a client whose work I'd just declined, precisely because it was above my level in that area. That client eventually requested help from me and sent me the atrocious work he received. I contacted the writer to tell him that he obviously had no business taking on any project that I'd had to decline, only to provide much worse work than I could have provided and I suggested that he owed the client a substantial refund, which he subsequently issued along with telling the customer that he resented his having involved me in the matter.
On the issue of legitimate writers vs. scammers, the "test" that you're proposing as though it's something novel is no different from what many of us have always recommended for new customers considering using any service provider: namely, always just try out any new writer or company with a smaller order and leave yourself a deadline cushion. That may not be possible if you wait until the last minute with a major project, but smaller projects allow you to minimize your first investment and the deadline cushion allows you to mitigate your risk if the worst-case scenario materializes. The same goes for your suggestion that your customers leave feedback here, because that's generally how this whole thing already works for all parties involved.