Word-of-mouth strategy only works when your intended target is more than willing to share his opinion with his friends and tell everyone about you.

The problem is (according to most companies i've worked for) most good customers will just buy your product and shut up. They don't brag about how good your product is; and it's likely their friends will never know about the purchase.

Bad customers however will tell 10 friends; and those 10 friends tell another 10 friends (and so on and so on). Semi-Myth / Semi-Fact; they are basically right. You can game this and try and win more customers from negative attention but it can backfire.

Lastly, not everyone looks for new products to buy. Some people don't know they need to buy something until you market them a problem; and the solution. If your expecting customers to magically just find you; maybe you will find money magically appear in your bank account.