Primary research can be a very good way of getting information, but you have to be careful. The first potential problem applies to translated texts; translators always put some of themselves in the works they do by translating certain words or phrases in their own way, which can change the meaning of the work drastically. But you also have to be wary of the original author's bias, so even the original language is risky. Reading a primary work allows you to make your own conclusions, to read between the lines to try to figure out what the author and/or translator meant; that is very helpful because you know you'll get exactly what you're looking for without it being affected by the input or bias of someone else. Primary sources can be exhausting because you have to take everything they say with a grain of salt; you have to question everything they write, think about why they wrote what they did. And yet they are the purest form of information.
Secondary research can be easier to read, and it makes less work for you because someone else has already gone through the painstaking effort of analyzing the primary source. It is also a good way to back up your own claims because it shows that someone else agrees with you. However, it can be difficult to find secondary sources on exact topics, particularly if you are researching something very specific. You also have to worry about yet another layer of potential bias that you have to take into account.
Overall, I have always preferred primary sources because they are the least removed from whatever you are studying, the closest you will get to an eyewitness account. I tend to come up with my own claims based on my primary research, and then do the secondary research to find others to back up my opinions. But you can also do it the other way around.