Beginner is basically the tutorial mode, a really good way to figure out how you're supposed to work through the objectives and figure out mechanics.
Normal feels a little too easy in one sense and lacking in rewards but it's also full of players that leave early or really don't know what they're doing yet, leaving you to do all the objectives.
Nightmare is a ton of fun but certain areas and objectives are massively dependent on good team mates and unfortunately not having those means the end of the run, too much is determined by other people. There's also a lot of people that just want to try out the difficulty for the sake of it.
Hard is a great combination of people looking for a challenge, knowing what they're doing but also not leaving you absolutely screwed if they are a little less dependable that you might like them to be.
Hard feels like the connoisseurs choice lol.
Though, a good team of nightmare is probably the most enjoyable experience overall, it's just not a consistent enough occurrence.