One can hope that - in the event you lose interest and move on to other hobbies - the good people over at Rawtherapee will have the good sense to incorporate many of your methods.
I appreciate your comment that the RT platform has gotten a bit cluttered to address legacy compatibility concerns, but there’s still hope that you are lighting a path for RT’s future as well.
Yes, I understood that – sorry for the confusion!
For me that is one of the strengths of Darktable. I do plan to watch your video on masking in Art and give it a try. I am impressed with what a single person has done with ART. At least there are no committees to argue with and you can just do what you want.
BTW, another strength of Darktable for me is the ability to do multiple instances of modules, especially exposure. Is this possible in ART?
Can you clarify what “modules” of exposure refers to here? You can open multiple RAW edits at the same time (at the cost CPU resources, of course).
But do you mean edit exposure of an image, and then open another exposure edit on the same image at the same time? If so, what is the motivation for doing so?
Through the use of “snapshots” (saved in your edit sidecar in ART, unlike RT) you can save multiple versions of exposure settings and flip between them for comparison. Perhaps that’s what you’re try to accomplish with multiple modules? 
In darktable one will often use the exposure module with a mask to achieve a dodging and burning technique. It is handy to have multiple instances of a module so you can use it with different settings and different masks.
This is a real strength in Darktable. In my opinion DT does it better than any other program I have tried so far including Lightroom. I will devote time to learning masks in ART. I like the pure simplicity and lack of clutter in Art. That is the strength of ART in my view.
You can have multiple “regions” in the local editing tools, each with its own mask and its own set of controls. Again, not quite as flexible as darktable, but a different trade-off (if you want even more flexibility than dt, you can go to a full-fledged node-based editor like Natron, for example).
Here’s a short demo video that illustrates the concepts. (The same applies to all the local editing tools):
HTH
Isn’t the ‘highlights / gain’ function in colour corrections, essentially the same as adjusting exposure (as asked by @Terry) just with a different scale. This can be applied many times to different parts of the photograph with masks.
I think another strength of masking in ART is the ‘colour similarity mask’ to select regions of similar lightness/colour. This is so quick to use, just sample with the pipette and adjust the range. Apart from the odd occasion, I find it much quicker than parametric masks.
Yes indeed.
I love these short videos as I learn so much from them. Thank you.
I often use a color similarity mask + brush mask to limit the area, for example if I want to brighten a face or bring down highlights in the sky. I use area masks sometimes too but have not used parametric masks yet.
Really? I’ve tried rawtheraphee several times on several computers and it was always so slow that I did lost my patience 
Maybe it needs some performance configuration?
Maybe all my computers have just been under powered…
None of the raw converters are really snappy when image ops are concerned but for me on every computer I’ve tried, darktable is consistently a little slower than RawTherapee.
That said, both are still very usable.
Very slim, to be honest, as I don’t have a use for it. But I thought something like gimp would be more suitable for this. What would be the advantage of having this is ART?
One less step to prepare the photos. DT has this module and it does speed things up. Opening GIMP to apply a watermark is like using a bazooka to kill a fly.
Not sure if it’s any help, but here is the meat of a script I use to apply watermarks with ImageMagick:
composite -gravity $LONGLOC -geometry +40+40 $WATER $IMG $OUTPUT
$LONGLOC is one of SouthEast, SouthWest, NorthEast, NorthWest. $WATER is the image file for the watermark text. $IMG is the input file and $OUTPUT is the output file. It’s certainly easier than firing up GIMP every time.
This can be scripted to be applied automatically on export from within ART. If that’s an acceptable solution, I can show one way of doing it.