There is good evidence that too much synthetic Vitamin A can be dangerous, but the evidence isn't as strong for natural Vitamin A. There have been some cases of overdose from natural Vitamin A, but they have all been from consuming organ meats from carnivores. Beyond the recommendation not to consume carnivore liver, there's not a whole lot of evidence that Vitamin A toxicity is really a likely problem to have.
Recently, there has been more evidence that Vitamin A needs to be balanced with proper amounts of Vitamin D and that excessive amounts of Vitamin A can exacerbate Vitamin D deficiency. Cod liver oil has been given a bad name through these claims, and the main reason for this is that the processing that removes toxins removes most of the Vitamin D and leaves most of the Vitamin A. People know that traditionally, CLO was used to treat rickets, and they falsely believe that any old CLO that they can buy in the store will help cure their Vitamin D deficiencies, and so they turn to these processed CLO's (many of which contain synthetic vitamins and/or almost no vitamin D at all) as natural supplements, which actually make their problems worse. Mercola's claims are based on this evidence.
I haven't seen any evidence that FCLO is bad for you. It was used traditionally by many cultures as a sacred and healthful food. If it were gravely dangerous, I would think we would know by now. FCLO contains huge amounts of both Vitamin A and Vitamin D and should not exacerbate problems with Vitamin D deficiency. In fact, it was used traditionally to treat rickets successfully. However, if you are not getting enough Vitamin D from the sun, it may not be enough to get you to optimum Vitamin D levels, depending on who you ask.
Rickets is caused by a very severe form of Vitamin D deficiency, and, as the NIH knows, if your serum Vitamin D is at or above 20 ng/mL, you won't have rickets, and many people have much lower levels without actually having rickets. However, most of the people that I have seen that are actually studying Vitamin D believe that optimum Vitamin D levels are around the range of 50-80 ng/mL. I believe that there's good evidence for this since that is the approximate level that people who work outside have, breastfeeding mothers have enough Vitamin D in their breast milk to nourish an infant born in the winter in the far north or south latitudes with those Vitamin D levels, and there are numerous studies that have shown that various health risks are reduced with those levels.
While FCLO could be great for raising Vitamin D levels somewhat, I don't think it is a good way to get all of your Vitamin D. You need to get it from the sun or other supplementation as well, and if you don't, you can expect to be deficient even if you are taking FCLO (though the FCLO would still be doing something good for you). I have run into too many people who have been deficient despite taking FCLO. Traditionally, people spent a lot more time outside, built up some extra Vitamin D in the summertime, and used FCLO to help keep their levels up enough in the wintertime when it is not possible to get enough Vitamin D from the sun in most latitudes in the United States. Traditional cultures from polar regions relied more heavily on seafood in their diets, which provided more Vitamin D.
There is a finger-prick and mail-in Vitamin D test available advertised on the Vitamin D Council's website.