The biggest problem with Google AdWords is that you can burn all your money easily and simply fail at it. It can be really frustrating when you do not see profit after you've sent way too much money on generating visitors.
One of the biggest reasons for failure is that people do not research their market well enough. They just skim through it and make their decision based on first impression. You should understand that for using AdWords, you have to be responsible person. You have to make sure everything is in its place before starting your campaign, or you might waste a lot of money.
Usually, people tend to think that they just have to start campaign to see the profit. They do not realize that there are other people fighting for the same keyword until it's too late.
But there is something that is worse than that – traffic that is not converting. Basically, you are paying for visitors, and visitors are not bringing you any money. That is the worst option.
The solution to those problems should be proper research before entering market, good keyword research and testing. You should test things every day and get rid of keywords that are too expensive and that are not converting. Also, try to add something new every day.
Another thing to consider is to start slowly with profitable keywords that will give you profit for sure. From that point on, you can slowly increase your business because you are already receiving some money.
Just like for pretty much everything else, there is a learning curve for AdWords as well. Only your commitment and hard work will determine your profit.