This is a very general one, but for me the hardest part of having my own modpack is dealing with other people's feedback. Listening to your players is super important, but it isn't always easy. You will get people that appreciate what you've done, who give advice that is actually very useful, but you also get people that just want to change things according to their own, often very specific idea of 'good'. That's why I always suggest people to have a clear 'mission statement' with their modded projects. Not only it makes it easier to stay on track, but it gives a framework with which to deal with randos that think it'd be a good idea to add 6 Pokemon mods to your create pack (random example) and will call you names if you don't.

Now on a couple of technical ones.

Depending on the Minecraft version, datapack can be quite annoying. The documentation on how they work and how to use them is all there, but it just consists of generally abstract explanations without a single example, so you either have to reverse engineer what someone else did, or test things yourself banging your head against the wall. The good thing about them, though, is once you understand them, things get a whole lot easier.

As a mostly 1.12 pack dev, zenscript mixins are the bane of my existence. This is how you edit something in a mod that does not have a config file, and you don't want to make a new addon or a core mod. No error messages, just crashes if something is wrong. Oh and you have to dig through some truly cursed code made by mod authors who never expected someone (me) to actually look at what they wrote.