The 9mm rim is smaller in diameter, so it is not tight in the breech face, and the extractor will be looser. The 9mm headspaces on the case mouth, and is shorter than the 38ACP/Super, so it can drop into the chamber too far. The way the Colt 1911 firing pin is set up, it can still reach the primer, as the firing pin can protrude quite far forward until the spring goes solid, or the taper wedges (wedging a firing pin forward is asking for unintentional full auto, not a good thing).
The ejector is set further rearward for the longer case, so ejection might not be reliable with thew 9mm. When I had a gun set up for both, with a 9mm and a 38 Super barrel and slide assembly for each, I cut a 9mm ejector halfway back between the 9mm and 38 length.
With the 9mm case chambered forward of the breechface (assuming the extractor is not holding it there), the case can recoil rearward and slam into the breech face too hard, which might not be good.
The 38 Super headspaces on the rim, small as it is,, some older barrels were set up to headspace on the case mouth, but they are not as accurate that way, I heard.
A 9mm case rim will fit a bit loose in a 38Super slide, but a 38Super case rim will not fit in a 9mm breech cut.