For hiring bad SEOer:

Your requested keywords can be ranked quickly.
You can have a new customer base.
The cost you pay may be less than what your competitors pay for SEO.

And the consequences:

• When Google discovers that your site uses black SEO to rank higher, Google has many ways to penalize your site. You can imagine how to penalize it, and Google is smart enough to know that you are tricking it.
• Your site will be completely deindexed from search engines.
• Once you realize this, it will take a long time for your site to return to search engines because you have to prove to search engines that the information on your site is really necessary for users, while if you don't use black seo, this is almost unnecessary.
• Lost customers.
• Hire a professional SEO to fix the errors at an exorbitant price.

Experience to recognize Good SEO:

• If you hear the phrase “we always guarantee your website to be on top” then don’t hire someone to say this. There is no guarantee about this issue and a good SEO understands this better than anyone.
• If someone tells you about the over-importance of Meta keyword tags, don’t hire them. Meta keyword tags are not no longer valuable on search engines, but their absolute importance is no longer.
• Ask the SEOer to give a list of sites he has worked on and check those sites, of course, the newest clients, based on the checked information then decide whether to hire him or not.
• Good SEO will have more advice on your site's content and structure.
• Good SEO will research and provide each correlation between keywords and site orientation for you.
• Good SEO provides regular reports.

Some opinions are subjective, will not be true in some cases, but the above opinion is probably the most common. Be smart in deciding to hire someone to do SEO for your website.