I am having a deja-vu moment here because I think I talked about this years ago. I was under the impression a moulder can't straighten a twisted , bowed and cupped board.
Generally speaking you are correct.
However, there are (relatively) modern 5 head moulders with a long infeed set up like a jointer, and the first head is a bottom head, instead of the tradition top head. They are designed to flatten but you are still at the limitations of any powerfed jointer mentioned in the rest of my post. For making mouldings and for generic squares(rectangles) they do a pretty good job.
Also, stock fed into a moulder probably came off a straightline rip-saw. So the edge is within a tolerable range of straight to start. Depending how the moulder is set up & fed, it can get better or merely stay about the same.
The old machines i mentioned and owned are designed/will flatten twist, cup and short bows effectively. They do not do well with long bows and i really doubt the ability of a power fed jointer to deal as well with long bows as a skilled, practiced person can on a hand jointer.
I too, thought you asked that in the past.
At this point if you don't believe or understand why my experience and observations are as stated, then you should take some 12' or 16' boards to one of the machinery shows and have them run the worst ones that you would practically expect to use and see what happens.
Twist removal is limited by the amount of thickness you can lose from the board; that is true for any method.
Hoever, if you don't take enough of a cut, or the board does not have enough thickness, the machine will feed it flat until the surface runs out, then begin pressing it over and start cutting again. Yielding sort of a 2-flat faceted surface with the flats at angles to each other. If you don't know what you are running and the condition rears up in later steps of the fabrication process, it can create time consuming nuisances or compromise the work.
Same with a long bow, escept the flats will be segments of long bows. or it might just roll it though, flattening side to side but maintaining the long bow.
This can all be avoided at the bust up saws of course, dividing "difficult" boards into short parts before they are flattened, exactly as one would for a hand fed jointer. What i am trying to convey is that the process can work fine, but it is very different from hand work on a manual jointer. It is not the best way to optimize expensive material for technically demanding applications if the lumber is long.
A tiny example: If you are straightening say a 10' long board with an S bow in it, knocking off the humps on each face will cause it to relax and flatten. Then you work the faces differentially until one is flat &/or you have used up most of the thickness tolerance. It will probably come out of the planer on targer, and still fairly straight/flat.
If you take all the material off one side, with the S bow having one hump up and one down, very little relaxation will occur at the concave down section, but the convex (down) section will relax a lot. so after that face is flat, running it in the planer will knock off the hump on the opposite side of the convex down section, and it will relax going concave up. The stick will resume its S curve in the opposite direction when it comes out of the planer, though it will be more a straight end, merging into a hooked end (in the flat plane).
This is why i will never order S2S lumber in packs. Too much of it will not be flat enough for long work, and there is no remaining thickness to make it so. If the use is primarily short parts, and the lumber is busted up into short blanks before flattening by someone who understands working between or among defects, power fed jointers/facers can work beautifully. (Short to me is under 4') Or, possibly, much work or process does not require lumber as flat as i prefer to start.
smt