Is the iMac sealed like the Macbooks? That is the significant fly in the ointment.
I personally think it is silly to 1)seal a computer from upgrade and 2) buy an all in one unit for the desktop. At some poiint, you would want to add or change RAM, Drives, Graphics Cards... But then, my computer is in a SuperMicro Full Tower Server case that about 28" high. Not sure anyone still makes them that size. It has 2 SSDs, 4 HDDs, 2 BD Driver, and Interanal Card Reader and even a 3.5" floppy drive still.
My laptop has has had RAM, HDD to SDD, new Radio (WIFI and BlueTooth) Card and Antennas.