I think I explained the "Hard to write for" situation pretty well in my previous post...
Kenneth Biller said that he thought it was a pity Kes left because there were so many possibilities for her given her unique lifespan and abilities, so I don't believe the writers couldn't come up with story ideas for her.....but it was easier to write lines of dialogue into episodes for characters like Chakotay and Harry because they could be given filler lines or technobabble lines each episode. Kes actually had to be given her own stories or worked into other stories and that obviously required more effort. I don't think it was a question of "can't", simply "can't be bothered".
So I do believe Kes was more difficult to write for than say Tuvok or Harry because they could get their quota of lines shoved in saying stuff like "shields down to 80%" or any random Starfleet nonsense. Since Kes was a more unique character, she was more difficult to write for. But the producers jettisoning Kes was not a testament to a character that had no potential/was too difficult, but simply to the laziness of the writers who were far more comfortable writing character types they'd already done.