If you are an online publisher,  an advertising network which is a  major source of revenues can decide to terminate its partnership with you abruptly, without assigning any valid reason. Similarly, websites can lose their traffic from search engines anytime. So it is always better to
1.Use a large number of ad networks for monetization
2. Have a large number of  blogs/ websites
3. Sell advertising directly

One of the main disadvantages of using PPC or CPM advertising is the fluctuation in revenues. The advertising network may choose not to display any advertisement or only show public service advertisements for which the publisher is not paid anything.

A business interested in generating sales and leads online can use the services of a specialized  Internet Marketing firm , which uses a combination of  Search Engine Optimisation,  Search Engine Marketing and Email Marketing to boost their business.

One of  the most striking features of having an online business is the amount of sabotage  you face.  Almost everyday, some one tries to  hack or DDOS your website.  So when your revenues are OK, always make a provision for times when your income is zero.

One of the most important criteria for selection is payment terms. Some networks pay almost immediately, others pay on a net 15 , net 30 or net 45 days basis. You have to adjust your cash flow accordingly.

It is always important to diversify your internet income revenue streams.   You never know when a particular company will shut down or  your account will be disabled for no fault of yours.

Most affiliate programs use cookies  for tracking conversions and sales, but cookie stuffing seems to be rampant.  The owner of the website who has actually made the sale , usually does not get any credit for it.

One of the major problems for Pay per click (PPC) advertising is click fraud.  You may be an honest webmaster , but there are a lot of website owners who want to make quick money and will develop sophisticated click rings.   Since the advertiser does not get genuine leads from these websites, they will also discount leads from high conversion websites.