Kind of a debate regarding technical accuracy vs practical usage.
Seems the Alpa and Arca are pretty much the same in where the "tilt" occurs, both effectively tilting right behind the focusing helicoid? As far as distortion, I would assume this means there isn't a technical camera that avoid this, especially when it comes to a little longer glass (I tilt with my 70 and 90 frequently when doing pano stitches), since the entrance pupil/nodal point is considerably in front of the tilt plane. This means that whatever distortion might be there, you can't avoid it unless you move to a true bellows focusing system where the tilt is at the lens.
I've never seen any real distortion, so while technically it might be there it seems insignificant to the end goal, and may only be able to exhibit itself with testing and special targets. This may apply less to other types of photography where manmade geometrical objects are in play.
I don't use an app to manage my tilt. I simply use the "focus far/tilt near" method, which to Yunni's point is extremely easy to do with a CMOS back, and to be honest I was pretty good at it with LiveView on my CCD back.
Also I"m not sure what is meant by the term "hold focus". Tilting definitely changes the focus on my Arca. It takes a few iterations of "focus far/tilt near" to dial it in, because as soon as you tilt, infinity loses focus and when finished the helicoid isn't anywhere near the normal infinity mark or near the close focus point of the near object in the shot.
I have had both systems. The Alpa can be much smaller, but having the tilt in the body is nice. Unlike Dave I like the helicoid system of the arca but admit I had the Alpa system before their new focusing rings. I haven't compared prices on glass, but logic tells me one focusing helicoid on the body should make the lenses assemblies a little less expensive than having to put one in each lens. My go to lens is the 40mm rodie but have used tilt on the 28mm rodie on occasion. Again with a cmos back and LiveView either focusing system is fantastic, and the Arca may be a little more cumbersome because of it's extremely fine pitch.
Finally I would agree that tilt is a valuable tool for some images with one of these cameras especially if the style of your work allows the use of tilt. Obviously as soon as you put tall elements in the foreground you have to resort to focus stacking instead, or you get the top of those elements out of focus.
From my perspective, shifting is far more valuable than tilting, it's actually pretty rare I don't shift a landscape image anymore, as I tend to keep my camera level and then use shift for the composition. Only when I hit the edge of the image circle will I then begin to tilt the camera to finalize composition.
I also do quite a bit of shift stitching, especially panos were I can get a nice format with a 3 shot shift on my 40mm.