If people have problems donning an immersion suit on terra firma with no need to rush they must be pretty cackhanded.
I can only go by what I saw and experienced. Entirely fit 30-year-old men found themselves trying to connect the straps at the side, only to realise that they hadn't put their head through the necessary hole. It was surprisingly awkward to get all the bits in the right positions, even having watched cabin crew go through the motions many hundreds of times. Of course, it's not easy to perform an unfamiliar task in front of twenty sniggering adults, but then the circumstances under which it would have to be done for real are not likely to be quiet and serene either.
Note how many of the passengers standing on the wings of Sully's Airbus had made a hash of getting their vests on - and they surely knew they would need them in that situation.
It really is a shame for aviation pros that their planes aren't entirely populated by highly-trained military personnel, but only us cackhanded, dithering, unfit, disobedient punters who have the effrontery to want to get to another part of the planet without walking.