It's almost always insanely time consuming.

It's difficulty depends on what you want to do.

Doing it with multiple people helps with the quality and scope of projects. Many people just mod to their personal tastes and never upload anything.

Development experience is a huge asset but is not required. Usually people without development experience focus on the building and art side of things. Those things are still very time consuming, but require less practice to get used to.

Depends solely on what you're doing in it. There are simple things like customizing item stats, changing their models and sounds and those are pretty straightforward things plus terrain customizing and adding your own houses and such are simple things to do but the quest crafting is usually much more complex but NOT if you're willing to put some time in it. I've modded with both the Construction Kit from OB and MW plus I've modded with the GECK from FO3 and FONV and I've self taught myself using tutorials on Bethesda's GECK site and I can tell you it's not that hard once you wrap your head around it. But if you want to start adding content like your own weapon or armor models or even your own hairstyles or face presets then you're gonna have to start learning software like 3DS Max or Maya so you can make your own models or customize existing ones but as I've said it's possible if you wrap your head around it. But 3DS Max nor Maya won't come cheap for you.

Depends on what you want to do, but in the FO3 and MW construction kits it was pretty easy to, say, add new dialogue and places.

It is easy if you want to mess around with some values/constant in the game (jump height, combat damage, spell cost) or change enchantments of existing artifacts or change racial/vampire powers.

It is more complex if you need to do animations or build a house.

It is even more complex and only possible with script extenders if your target is outside what the game engine is designed for. Examples include blocking while dual-wielding, sleep to level up, bypass perk requires, recreate attributes, spell making and so on.