I think one might need to look at that on a function-by-function basis; with respect to demosaicing, for example, I don’t think anyone, FOSS or commercial, compares to the performance of RT…
If I hadn’t done my own thing, I’d likely be using RT to develop linear TIFFs to take advantage of all the raw goodness there, and opening these TIFFs in darktable to do the color and tone transforms…
Hi @paulmatthijsse, absolutely, there have been many extremely valuable contributions to RT! @Hombre, @agriggio and many others. These graph does it better justice: Contributors to Beep6581/RawTherapee · GitHub
I did not mention them, since their last direct contributions to the project were mid 2019 or earlier, so I don’t consider them ‘active’ contributors at the moment. Of course that may change in the blink of an eye. For example, lately Adam Reichhold has returned to contribute again, which he did in 2016 for the last time. Never say never 
Thanks for all the responses. I use RT for a lot of raw development (Fuji and Nikon), sad to say it doesn’t support my Sigmas.
In many ways much better than anything available.
Very strong points: Demosacing for Fuji, sharpening (contrast by details). Now that I think of it IDK what you could add. Layers? IDK, Brushes for some of the tools?
If I could make a humble suggestion, it would be that it is at times overly complicated. I think several controls have little or no effect and other like wavelets are difficult to master.
Many thanks to all. I am an Oracle DBA and C++ coder btw.
Foveon or non-Foveon?
Foveon has some really unique challenges that make it extremely difficult to support. It’s significantly harder than X-Trans (which gets supported because a few RT developers do shoot Fuji on occasion, I think most X-Trans support has come from contributors who own the camera.)
I have a few tiny contributions to RT myself, I keep on intending to contribute more but I have this bad habit of hopping between various projects, it’s only recently that I started doing photography/videography stuff again after quite a long break. There’s literally one github issue where @Thanatomanic provided me some data to look at 6-9 months ago and I STILL haven’t gotten around to grabbing the files to see if I can notice whatever phenomenon he was describing.
When you describe the circumstances / process, it really is impressive that such a great piece of software emerges from it all!
Seems like darktable currently has the most rapid development, but I find RawTherapee / ART still have some of the best quality algorithms, especially for sharpening and detail.
Yeessss!! 
I have 2 Foveon cameras, SD1 and DP1 and I have the SD Quattro. RT will process the files from the DP1 but the corners have a weird cast.
I cannot find any RAW examples online for the camera types you mention so maybe this might be a Foveon issue, but my guts tells me this is probably a lens thing.
Are the lens(es) you use supported/present? Transform tab → Lens /Geometry → Profiled Lens correction
And have you tried using the Vignetting Correction , also in the Transform tab → Lens /Geometry section?
Sigma RAW format is X3F and last time I checked RT (or DT) could not handle them. In fact, not many editors can.
I’ve read that in the corners, Foveon sensors get crosstalk from angled light.
Just checked the libraw supported cameras list, and all three of the OP’s cameras are listed (although libraw lists “SD 1 Merrill” and “DP1 Merrill”, don’t know if “Merrill” indicates a limitation). Now, I downloaded a DP1 Merrill raw from dpreview, .X3F file extension, and tried to open it in rawproc with the latest libraw, and it didn’t work…
I did find some X3F examples [sigmaphotopro] and they don’t load (are ignored) by RT, DT and Filmulator.
The xtrans demosaic was coded by Frank Markesteijn for dcraw.
Then I started to port it to RT and at the same time Dan Torop from darktable ported it to darktable.
That was a very nice collaboration resulting in a much faster implementation than the one in dcraw.
Doing this, RawTherapee got a lot of help from @sguyader by providing test shots and also testing to get a good (though not perfect) xtrans support in RT.
At least I never used an xtrans camera 
I have a Sigma SDQ as a pet, not as a every working camera. Foveon is excellent in things they excel, and terrible in all other aspects. In the past I used the (free) Sigma Photo Pro (a terrible slow software packed in a 2003 design), but since a year or Affinity Photo speaks X3F and I stay there.
IMHO, as a Sigma user, it’s such a niche format and with no new model out since 2016 I prefer RT/DT developers to do other things, like play with their dogs and stuff. If I REALLY must use DT/RT I can load the Sigma Software, batch convert X3F > TIFF, have a rest, and then use i.e. darktable or rt. Really no big issue.
Here is a X3F I did a few weeks ago: (50Mb - interestingly google can render it) _SDI0153.X3F - Google Drive
Isn’t the SD Quattro capable of writing DNGs?
Yes, I tried it and didn’t like it. May have to try again. The beauty with X3F is that you can take an image and later decide you want it color or monochrome. A monochrome DNG will stay monochrome (I think).
It was a great pleasure to get involved in this process, and to help at my humble level. RT devs are nice guys!
Just my 2 cents but I think RT would benefit from more frequent releases. New releases generate publicity and hype, and the nightly builds have plenty of new stuff. The vector-scope / parade alone would justify a new release in my opinion.