I use it on my Kindle and I love it. I can use better and more dictionaries, the controls are ergonomic and customizable, I can easily override the font so that every ebook looks exactly the same. It supports epub and with it my Kindle is faster, the battery lasts longer and it supports dark mode on an old model while Amazon officially doesn’t. Amazing software all around!
While you're definitely right about it having a steep learning curve, it's also true that not many RAW editors do what Darktable does. Darktable aims at serving advanced, tecnically-minded users. It's complicated, but in a way, it's nice it is, for those who need it.
To change modules order in Darktable, you have to know what to do. By default, the module order is correctly arranged and no intervention is required. Changing the order is an option that you can use only if you want to. No user will ever change a module's order by mistake (and in the remote case that one did, fixing that is a click away).
We have Lightroom, we have Capture One. Let one software for more technically-inclined photographers exist, where you have options and power over the pipeline. Darktable does not aim to be a Lightroom replacement.
In general, I do think that Darktable's UI could be better, and most likely it could be more efficient. His points are not wrong, but they are also not as fundamentally disastrous as the author makes them to be. I feel the damage that the author caused with his behavior is potentially worse for Darktable than the problems themselves.
In a collaborative project run by volunteers, the way one argues and the point one makes are united. Darktable is possible only if there is a group of people working on it. I've seen the author insulting fellow contributors in public forums, I've seen him using the most offensive possible language against people who were being polite to him, just because they disagreed. This is not how you steer an opensource project in the right direction. It is also useless to come and shit everywhere after the changes are done. It is the opensource equivalent of seagull management [1].
I think in a collaborative project, the way you state your point is as important as the value of the point you're making. If you're unable to state your point clearly and effectively, then you should work alone, as the author has now correctly decided to do. Continuing the shitstorm even after his collaboration with Darktable has terminated seems, to me, to be just the continuation of a useless feud.
I say all this as a happy Darktable user, and not a contributor at all. I also loved the author's contribution, and found his Youtube channel to be really informative and a fantastic learning source. Still, Darktable has continued after his departure, and the new release, which will come for Christmas, is really promising, with the inclusion of per-channel curves in the sigmoid module, similarly to Blender's AgX. Many other improvements are coming, and I don't feel the project is "crashing into the wall in slow-motion".
The author, while having contributed greatly to Darktable in the past and being without any doubt skilled, has severe communication issues. When he was in the darktable team, he was abrasive and offensive. There is a limit to how much offensive you can be to others, parlrticulaly when you are all volunteers. So, I'd take his words with a grain of salt and take into account the background of all this: a skilled person with a very little ability to speak to others starting a feud with another opensource project to which he contributed in the past.