Perhaps I am misunderstanding it, but I don't see anything wrong with emphasizing the personalities of the people in history, not just the processes. I am not saying that it is the only way to view history, but I think it could be yet another valuable lens along with looking at processes and movements, and other things outside of the personality.
There's nothing wrong per se with focusing on specific people, and doing so is a perfectly valid means of examining history. Especially military history, where sometimes it really does come down to some guy. Or someone not getting said guy's plan and botching the job. (Goddamnit, Ney.) But it's not just that, see; it's the specific focus on such Great Men, usually to the exclusion of all others, that makes it iffy.
Further insight on this particular matter is always appreciated; for the meantime, here's a few previous threads exploring the Great Man theory:
As per usual, all of u/DanKensington's excellently linked threads merit reading, and my response will focus more on the research-specific problems of the Great Men theory and what dangers it leads to. Let's begin.
The bottom line with the Great Men theory is that the course of our civilisation's history has been the word of Great Men who possessed a particularly quality which enabled them to drive forward the wheels of history and progress. It's become customary at some point in learning history (most notoriously at the high-school level, or in the troves of "pop-history" videos and articles) that some homage is paid and some praise is sung for such men: Napoleon Bonaparte, Genghis Khan, Winston Churchill, and George Washington to name a few. Of course, once you get into college-level academia and beyond, the scholarly depth of works means that highlighting the actions of one individual seem rather dangerous and ad times downright contradictory to the larger circumstances or movement which they were merely a figurehead of.
Case in point: Kaiser Wilhelm II, Kaiser of the German Empire, 1888-1918.
"Now hang on a minute, who on earth is calling Wilhelm II a Great Man?" I hear you ask with some disbelief, well the quite simple answer is: no one. Instead, the last Kaiser of Germany is an excellent example of what happens when we take the Great Men theory and flip it the other way around, portraying the tragedies of human history as resulting from the actions of "not-so-great-men." In my research on the Origins of the First World War, no historiographical argument or scholarly overview is complete without taking some words to lambast Kaiser Wilhelm II's incompetence as a military leader or as a warmonger who knocked down the dominoes leading to continental war.
What we forget to look at and explore is how Wilhelm II was an archetypal product of Europe in the late 1800s. A prideful monarch at the head of a rising nation state beginning its era of hegemonic power over "lesser" nations whom it had just been victorious against (read: France). In such light, Wilhelm II's shift away from Otto von Bismarck's general policy of "balance in Europe" towards *Weltpolitik (*world politics) and pushing for Germany's rise as a world power seems appropriate and almost expected for a man of his time and position. Even more devastatingly, we forget to look at the influence that the Prussian military (later the Imperial German Army) had on the course of the First World War. It would be Ludendorff and Hindenburg who would come to control the military and state apparatus later on in the conflict, with Wilhelm taking a secondary role (though his support for the army could almost always be counted on in disputes with the civilian government).
Now of course, there are also benefits to the Great Man theory. For one thing (and the main thing): it's probably one of the best ways of getting younger audiences (and even more seasoned ones) interested in history. I imagine telling a 7th grade class about the French Emperor who conquered much of Europe would make for a greater entry point into the Napoleonic Wars compared to telling them the story of a Mr. Barker who forged horseshoes for the thousands of war horses which charged at Waterloo (fictional name obviously).
In short, by focusing too much on the actions of one man (great or otherwise), we ignore the larger circumstances which may have contributed to their actions, and the other figures who surrounded them and were also key stakeholders in the decisions of the day. Using that great man as a figure to then spark greater interest in said circumstances, movements, and conditions is a great strategy however, and one which the theory lends itself to.