Unethical search engine optimisation (SEO) is exactly that: morally questionable (or outright reprehensible) activity to make a website more appealing and easier to understand for search engine crawlers. Whether you are interested in morality or not, breaching webmaster guidelines provided by the major search engines can cause you all manner of problems.
First and foremost, there are serious legal issues involved with the most unethical search engine optimisation practices. Hacking into and manipulating a website on an authoritative domain can produce extremely rapid and impressive ranking gains, but these are never worth the associated risks. As we move forward into the digital age, computer crimes are treated ever more severely by governments around the world - unless you like the sound of a custodial sentence backed up with restrictive conditions of release, such tactics should be avoided at all costs.
Assuming you are not guilty of anything quite so serious, search engine operators have powers to make you regret investing in unethical search engine optimisation strategies. Broadly speaking, there are two different types of penalty that search engine operators can bestow: algorithmic penalties and manual penalties. Algorithmic penalties are automatically applied to websites when search engine crawlers discover a breach of webmaster regulations; likewise, they are automatically removed when the matter is resolved. Manual penalties are imposed by a human being and generally come with an 'expiry date' (provided all problems are resolved by that time). Webmasters are free to request another manual investigation before the expiry date arrives, although search engine operators are often so inundated with requests that this can be impossible.
Although search engine operators guard their secrets closely and the precise nature of the penalties they apply can be difficult to discern, it seems likely that most algorithmic penalties take the form of a ranking filter that forces the website concerned to appear lower in the search results for certain key terms. Manual penalties are more likely to be applied in cases of reported spam and other offences that can be difficult for computers to detect - these are most likely discretionary depending on the nature and extent of the offence for which they are given.
Even if your unethical search engine optimisation strategy does not breach the law and goes undetected by search engine operators, engaging in such practices can put the reputation of your business at risk. Web users who stumble upon meaningless machine-made articles, stuffed with links to your website, may well put two and two together and come to the conclusion that your business is involved in unsavoury promotional techniques. Although it is very difficult to measure the extent to which reputation damage threatens traffic and sales figures for your business, this is clearly an undesirable situation.
If you are serious about improving the visibility of your website in the long term, you should steer clear of any and all unethical search engine optimisation techniques. Unfortunately, insisting on ethical techniques can quickly add costs to an SEO strategy - but this should be seen as an investment in sustainability rather than an additional expense.