I don't think prequels are inherently bad, but I do think they are harder to write. Its a bit anathema to the normal narrative experience, I tell you about a person....and then I tell you some more. Its not as often to go "okay now let me tell you about his past now that I finished the current story".
I do think consistency of tone is the number 1 place a lot of prequels go wrong. A prequel has to accept the box its in, both in terms of what happens later, but also the tone and theme of the works that have come before. I agree King's Man is a great example of this going horribly wrong, the prequel did not feel like a "Kingsman" movie to me, it felt like a completely different beast.
The other tricky part about Prequels is.... what question do I want to answer? With sequels there is an obvious, "what happens to the main character now?" But in a prequel we already know a lot of that. So what purpose is the prequel trying to serve. Maybe its filling in the gaps of a backstory. Perhaps its adding in the "why" element as to why the main character ultimately does the thing we know they are going to do. There are a number of ways to go about it, but I think its very important that a Prequel nails this question, because otherwise they can tend to meander as compared to sequels.