Rabbi Shmuel David Luzzatto is a pashtan and grammarian. He is not a rationalist, though sometimes his conclusions are the same as the rationalist meforshim. But, if he thought that the best peshat in a pasuk was that magic was real, for example, he would endorse it as such.
He dislikes and criticizes derash, when intended seriously as historical and literal meaning of the pasuk. He dislikes and criticizes the 'philosophical derash' of Ibn Ezra, Abarbanel, and Rambam, as well.
He will consider dikduk, nikkud and trup, though he does not consider nikkud and trup to be dispositive. (If you want to see the intersection of trup and peshat, he is an excellent source.) He also will cite and consider the opinions of non-Jewish Biblical scholars, and of historical documents, and of the Peshitta. While he suggested some limited emendations of the Biblical text, this was not on Torah, only in Nach. And he criticized his contemporaries for suggesting emendations of the Biblical text, because he thought that they were idiots and ignoramuses who were not applying the methodology correctly.
He is certainly not 'mainstream' in the yeshiva world, for a variety of reasons. Open criticism of the Zohar and kabbalah in general is certainly one component of it. (I would note that people don't reject the Chasam Sofer for asserting that the Zohar is a forgery, even though the Chasam Sofer's father presumably believed in the authenticity of the Zohar.) Mental association of Shadal with maskilim also contributes to it, as well as his focus on peshat over derash, would also help.
But the Rav cited him on one occasion. And Nechama Leibovitch cited him, I think. There is an idea, put forth by the Rambam, of שמע האמת ממי שאמרה. So even if he is not 'mainstream', if his ideas are good, I think we should consider them.