The trouble with SEO's is that most of them are full of shite.
In addition to email spam i also get letters in the post that a full of rubbish. One even printed out a screen shot of the website talking about possible duplicate content. A line of text contained in the body of over a 1000 words was also repeated in the sidebar, which according to that toss pot was having a negative impact. I wonder how many unsuspecting victims he is now sucking the blood from?
Seo is a unique area of expertise in the sense that anyone can claim to be one and have
no accountability once things go belly up, some even set up companies offering SEO when they don't even know what a h1 tag is. It is a mine field for any business to find someone who can actually do it and do it well.
You can't even trust the companies that rank on page one of google for 'seo' to do a full and proper job - and they cost a small fortune to use.
Generally, paying more means better

- at least it does for everything else bar websites and seo.
The whole industry has been on its head ever since the word seo was invented.
Non teccy people can even be duped by referrals / references.
I appreciate that there are a handful on this forum that know what they are talking about, but i can only appreciate that having a great deal more knowledge than the typical business owner who is just waiting to get shafted by snake oil unless they get lucky and find one of the 1% of people offering the service that actually know what they are doing.
I agree people should look for results in peoples work but even that in the world of SEO isn't sufficient. You can rank sites top of google pretty easily for competitive terms with a sufficient know-how and a reasonably small budget, whether the rankings will last longer than a few months before you find yourself languishing on page 8 of google is another matter. So it's not like you can compare the SEO due diligence process to people asking whether so and so fitted them a nice bathroom - because the bathroom will always be there

the rankings wont.
I also appreciate that many small and medium business owners don't understand the true cost of decent seo, and as a result, to some degree, are getting what they pay for.