I have read in many of the answers in Politics.SE references to the SPLC. The assumption is that SPLC is a prestigious enough organization that their information is authoritative. It is not just here on this site, but both celebrities and politician also grant the SPLC authoritative status on their articles regarding anything related to hate speech or hate organizations.
On the other hand, There are articles (referencing opinions sometimes) that show that people and organizations have been falsely added to the SPLC list of hate speech. I am aware that anyone or any organization may say what they will and it is not necessarily true that they don't belong there
My personal experience is that I have not found anything in the SPLC website that is immediately false. In fact, some articles list hate-speech from people and organizations that one would think are liberal, including liberals that became nationalist white supremacist.
In the end, I am not sure how much to trust an organization that has a political agenda. However, if there is one, I don't know what the polical agenda is for the SPLC. Are they simply trying to get the word out but do not want to influence policy? Do they want to influence policy to help society?
I was not clear enough as pointed out by the comment. Wikipedia is an example of an organization that exists for information. Some of the information listed there is incorrect, some of it is political, but there is no intent on influencing political agendas. NRA is an example of an organization that uses information for a political agenda. Now I am pro NRA politically but I can see that the information on their articles is picked to influence the agenda. The SPLC role is not clear to me. Is the SPLC more wikipedia or NRA?