I've heard from some people that Episode 2: Planet 3 has the worst jump in the entire game with some even saying it's impossible. I wanted to know, what's so hard about it? I've never played Jazz Jackrabbit 1, only Jazz Jackrabbit 2. I want to also try Jazz Jackrabbit 1 soon on my PC.

TLDR; Why is the Episode 2: Planet 3 jump in Jazz Jackrabbit 1 so hard?

The second Orbitus level indeed had an incredibly hard jump, which kept changing a bit with subsequent patches to the game. Fortunately, nowadays if you get it from GOG, the jump isn't a problem anymore, because it's been fixed. If you want a lot of details about the patching process you can read through this thread (perhaps only focusing on posts with the word "Orb" in them).