A revolver IWB can be as comfortable, if not more comfortable, than an autoloader IWB if you have a good holster and not just one of those dime-a-dozen cheapie holsters. The shape of the revolver makes a difference for the reason Skribs points out.
I carry an old Rossi 971 2 1/2" .357 mag IWB and it is much more comfortable than my Block 19. That old Rossi is significantly larger than my J frame 642 being my Rossi is a K frame clone.
Heck I even carry a 4" 686 IWB just fine. The barrel length is at the max limit but the cylinder diameter is not any more issue than the K frame cylinder diameter.
A J frame to me is just too small to even try to make an IWB for. It fits in pockets but not all pockets. I wear blue jeans 95% of the time and there's no way I'm getting a J frame in those pockets without discomfort or the grip sticking out. Dockers and dress pants tend to hold the J frame fine for me.
Having said all that, I'm only pointing out relative comfort. The G19 is comfortable for me and I wear it when I'm all day in and out of Mastercraft suspension seats when four wheeling. I still daily my G19 but that's a round count decision, not a comfort decision. If I need more comfort or easier to conceal, the 971 definitely meets that requirement.
The issue for a decent size revolver IWB is finding a company that will build one. Seems to be too much bias against the revolver cylinder simply due to width without consideration for all the other dimensions that also count.
Andy