Here's a very lengthy post about the my experience with the UT community and the problems I've come cross. If you're in a rush, then there's a pinned comment with a tl;dr below. I'd suggest you read the whole post as I go in detail about specific problems.

Being a maker of fan-works, I've made connections with UT AU creators big and small. I love to help out people whenever I can, no matter how popular your project may be. I've been in the UT community since 2016. Some old followers may remember me for my first Undertale fangame, Elementaltale.

Ruins are Foresty! - ElementalTale - Undertale Fan Game - YouTube

It's just as bad as it looks. Your typical bad first project made by a kid. I've met several awesome creators that helped me improve my skills overtime. I was surprised about how openly-friendly the Undertale community was. I formed many friendships and helped out with countless fangame projects. I truly felt like I belonged somewhere. And now here I am, making a really ambitious Undertale fangame, True Geno.

...

Did you catch on yet? Did you notice I said "was"? Well, here's the thing. I've been in the community for a very long time. Over the years, the community has been slowly turning from friendly to toxic. I've seen it all, and I want to discuss those problems with you and how we can do better!

I've always wanted to give back to the community ever since I fell in love with playing Undertale. I'm constantly thinking of AU scenarios that I'd love to expand on. What if the UNDERGROUND was expanded into 3 layers and we explore the uncharted territory? What if Alphys figured out how to travel to different multiverses? Etc. etc. What other option then to make a fan-fic exploring these ideas?

Suddenly, you start to get popular and people show actual interest in your crazy ideas, going as far as to make fan-works off of YOUR fan-work. Awesome, right? In the beginning, it's very cool! ... But later down the road, well...

Disrespect Towards Fan-works:

Some people have the mindset that you don't own your specific fan-work, and their evidence to back that up is "because your AU is based off of Toby Fox's Undertale, and so that means Toby owns your AU". While Toby do own most of the characters and settings, Toby does NOT own all of the new assets created for your fan-work.

Because of this mindset, people assume that they can do whatever they want to other's AUs. For example, a fellow creator, Dorked, has been constantly getting discouraging messages like "Inverted Fate is just an Underswap rip-off". She has explained many times that Underswap and Inverted Fate are two separate universes! Here's a screencap of an FAQ section on the IF official Tumblr account:

My friend, Sensational, especially gets these types of comments on his fangame, SS!Underswap, a take on Underswap. Sensational often gets comments like "you're just ripping-off TS!Underswap. so unoriginal" TS!Underswap is a popular take on Underswap.

People seem to have this mindset that just because this popular fan-work did it first, every other fan-work similar to the original is automatically a rip-off. That's a really toxic thought process, not giving other creators a chance to show people their creativity.

AU Takes And Misconceptions:

There are a ton of creators that simply don't want people to make takes or fangames of their AU, which is fine! I'm one of those people that wishes to not have takes on True Geno. My personal reason as to why I don't want people making stuff like "Some Guy!True Geno" and "Underswap: Real Genocide" is because I feel like it's disrespectful to me to just have someone take my ideas, edit a few things, and call it THEIR take. I'm sure many other people feel this way, too.

A big example of a creator's work being disrespected is Voltra's Storyshift, the first AU to introduce shifting character's roles. Voltra is constantly in drama about people making fangames and takes of her AU. Taking a look at the Storyshift Tumblr blog, you can see that Voltra made a public statement about the rules of making fan-works of Storyshift.

Edit(March 30, 2021):

You can also see this post where Voltra go into detail about copyright of her work. One thing that stood out to me(and for good reasons) in said post is her wanting people to remember Storyshift for everything that Storyshift has to offer, and not genocide Chara and Asriel fan-fights. Give Storyshift a read! It's worth your time.

Another example would be Undertoad, made by LucasPucas, Patwhit, FMS, and other collaborative devs, an AU that crosses the Super Mario series with Undertale. Pat has be getting major disrespect thrown towards him because people keep telling him what he should put in his AU. Such examples like "you should put Waluigi in Mettaton's role" and "(non-canon song) should be Mario's genocide battle theme". When Pat addresses issues like these, people think he's being mean when that is far from the case. He's just simply tired of people making shameless Mario Sans genocide battles and fanon content. It's getting to the point where AUs of Undertoad gets hundreds of thousands of views while Pat only gets a few thousand.

The Abundant Amount Of Genocide Sans-centric Content:

This may be a hit-or-miss to some people, but I think most of us can agree that there are way too many genocide Sans and Papyrus battles. Ranging from Dusttale AUs, Revenge: TUE and Disbelief takes, to multiple Sans and Papyrus fights. Most of these battles over-shines some of the more original fan-works. Most of these genocide Sans battles devalues Sans as a character as he's not some 4th wall-breaking guy that can make the screen rotate and teleport to different universes, he's a jokey lazybones. Because of these battles, the community has become jaded over genocide route content. This community turned a single fight into an overblown spectacle, thus causing this "genocide hate" bandwagon. This is especially discouraging me because I'm often getting comments like "genocide route is overrated" and "true geno bad because genocide route". Comments like these often make me question my abilities and demotivates me.

There are more stuff I can talk about like the UT SoundCloud community, but I'm not well versed in that particular community.

Now, how can we fix these problems? Well, first, don't judge a book by its cover. Accept that there are going to be fan-works similar to a popular take and stop comparing AU takes. Inverted Fate can co-exist with Underswap, Underswap takes can co-exist with TS!Underswap, and etc.

See through people's ideas and don't tell creators what they should do with their AUs. Instead, give them feedback and suggest your ideas for an AU. If you don't like an AU creator's concept, don't make a take of their AU and call your take the "better version". Instead, make your own original AU with similar ideas. Look at Turned Reality for example. It's an awesome upcoming fangame with a concept similar to Storyshift!

And lastly, don't be afraid to step out of the boundaries. Utilize your creativity and willfulness to create something new and never-before-seen! Sans, Gaster, and Papyrus are not the only monsters in the UNDERGROUND, community. Don't be afraid to even make a fangame based on your OCs! The only limit is your imagination.

(footage shown is gameplay of UnREDtale and Battle With a Think Tank(Celeb) respectively)

I know we can do better. We all just have to try. I should probably say that this post wasn't meant to cancel anyone or something. The purpose of this post is to inform people of current issues and ways to solve them.