Well not really Criticizing but more like warning it's a scam. For example, I see an ad on TV about a website that will make you thousands overnight, and I write about how they are ripping people off. Could they come after me with a lawsuit? What are some ways to protect my site and me from getting in trouble?
The basic principle in libel law is that you cannot be successfully sued so long as your statements are true. Be careful of stating as fact that which is only your opinion; for example, if you state, "I know that these people are scammers," you could be running into trouble. "I think this program is a rip-off," is safer. Aside from the law, your target might get angry enough to report your site to your host, to Google, or malware listings, depending on how unscrupulous they are and what they can catch you doing. For instance, if you are running Adsense too close to your images. Using whois privacy on your domain is a good idea, namecheap includes this for free with transfers. Making enemies online is never good. If you believe they are unethical in business, they might be unethical in firing back at you. On the other hand, you might help warn others. That said, you are free to express your opinion on any subject whatsoever so long as you do not state falsehoods.
I don't think they can come after you for exposing the truth. Infact as more and more people will find out about them, their scheme will fail.
I had a hard time getting past your first sentence. It is simply not true. While truth may be a defense, it does not keep you from being sued. Having a defense and not being sued are two very different things. Simply, you can be sued for doing this type of site. Whether or not you are able to win in a lawsuit is another question.
Most of the stuff sold on infomercials is a rip off. They are banking on poor, unemployed people staying up late at night to buy their garbage and expect to live happily ever after. In America you can be sued for anything. Maybe there won't be a snowball's chance in hell of them winning, but they will cost you legal fees. If you are lucky you can recover the legal fees from a bogus lawsuit. I really would not worry too much about them suing you. Chances are their product is pure garbage and would not withstand the stink test...
You need to consider the people behind these sites too. Some of them are very shady characters. Years ago I made some posts exposing a scam and it turns out the people behind it were convicted felons. In addition to posting death threats, they filed a lawsuit to get a court order to turn over my ip address which would eventually give them my name and home address. Since you never know how serious someone is when making threats, my name and address isn't something I wanted them to have. I was also an officer in a public company at the time and didn't want to drag the company name into this lawsuit. I wound up having to file a motion to quash, which was successful, but cost me around $10K. Luckily they were arrested by FBI and were sent back to prison for the scam they were running. No matter what you do, if you put up a site, they can find you if you they really want... unless you are prepared to spend some money on legal fees.