The disguises were also a riot. A mask/disguise might work well enough for stage or screen, but I question whether you could pull it off in close quarters.
Ha yeah, even as a kid I had a hard time believing that trick could really be pulled off. What with people's very different bone structures and all the incredibly subtle details that make up the human face.
Even artists who spend weeks sculpting wax figures from exact measurements have a hard time making it look completely real.
Just on translucency alone, latex makeups would never pass muster in person. I imagine with gelatins and now silicone appliances you could get close enough (though there might be an analog equivalent to CG's uncanny valley with this too), but back then, no way.
Plus when you start realizing these people are running through water and getting punched and all sorts of other stuff (Harlan Ellison's VOYAGE ep where the network guy insisted the female spy be revealed as hideously ugly beneath a beauty mask, even though she has been aboard for months and presumably sleeping with crewmembers, leaps to mind.)
One con on M:I I recall that needed to be better than it was revolved around simulating a nuclear aftermath. The guy was underground, looking outside via periscope, and they basically built a little cyclorama with torched model buildings and what I recall as cigarette smoke, but it must have been more elaborate than that! Anyway, it did not convince or impress.