As someone guilty of this sometimes real life gets in the way. In my case my the death of a close relative messed up my muse. Attempts to get back into fanfic writing by doing silly stories fell flat. I do feel guilty but if the creative juices are not there, then what can one do?
I'm also guilty of this and I apologize to anyone that was really into a story that I didn't finish. For me, sometimes you have an idea and you get excited about it and want to share it, but stuff changes. It could be life gets in the way, you get lost in the story, you start seeing a different direction, it starts not working for some reason, and you drop it.
Recently I've completed works before posting them, but the last couple stories I've gone back to posting them as works in progress, in part because I do get excited about what I'm working on and want to share it. That being said, I also have liked posting after I've finished a work because that way I can go back, look at it again, refine it, and then put the completed thing out. The more I think about it, for me, I should probably just complete the longer works before posting them. My issue is that I start out thinking an idea is a small story, but it morphs into something much longer, and then I'm stuck in it, and hopefully can finish it to see where it goes.
I also use some canon characters, but I generally have used them sparingly. And it can make me anxious about getting their voices right. I do prefer using lesser known or fleshed out canon characters (example: Aquiel Uhnari) as well because there's more room to put my stamp on the character, while those characters are still rooted into Trek. As I've been writing for a decade plus now the small universe syndrome stuff doesn't bother me personally. I like being able to tie things together if need be for the sake of my stories and personal tastes. It is the Trek characters and the stories they've been in that drew me to the franchise and inspired my entry into fan fiction writing, so why deny myself a chance to join in and play around with some of those characters and events that I enjoyed watching?
Personally I'm not much into reading Trek fan fiction adventures focused solely on canon characters, and I do prefer reading more about original characters in the Trek universe (insert plug for our excellent United Trek fan fiction universe here), but if there are readers who are into that, if that works for them, I'm cool with that. I mean, Trek fan fiction writers aren't getting paid for this, and readers are also spending their time reading this when they could be reading licensed Trek, watching live-action Trek or TAS, or even reading something else entirely. Because of the informal nature I think there are going to be more bumps in the road, which is unfortunate, but I do hope it doesn't turn people off from continuing to read fan fiction.
I also have no problem with people having criticisms of fan fiction. I believe people should be critical of everything we consume. Though as a fan fiction writer I do prefer constructive criticism more than just being negative or insulting. Thankfully, over the years, the criticism I've gotten has been constructive. I haven't always agreed with it, but for the most part, I think the vast majority was constructive, gave me something to chew on, and ultimately helped make my writing, characters, etc. better.