RRFS has kinda summed this up for you.
Some other disadvantages are that Apple keeps closing and tightening their grip in their universe.
Meaning that they tend to now want to keep their loyal users buying every 3-4 years as Apple significantly upgrades technology. As a long-time Mac User, I have been noticing that Apple obsoletes perfectly good Macs (especially iMacs and lower wnd Mac Minis) every 3-4 years, or so. Mac users are not getting the same longevity out of the Macs as they once did. The excption is the Mac Pro Line. But other than serious professionals, who can afford one of these new. The Mac Pro line of towers seems to stay in the game, at least for another 2-3 years past that of the iMac line, IMHO.
Other reported issues here in these forums (but still only reflect a fraction of user issues with the current iMacs) is the greater heat issues they produce. The air ventilation on thses iMacs isn't the greatest. Even my 2009, 27 inch screen iMac runs hot enough where I decided to run an additional, small fan behind it to further cool the iMac's aluminum case. Apple packs so much into an iMac case that nothing really ever gets the needed proper ventilation. Again, IMHO. Users of the current lineup of iMacs have also reported premature or early HD failures as well as problems with the iMac's LCD screen.
I bought my iMac used with the original 1 TB drive, (knock on wood) the drive is still woking fine and the screen on my iMac hasn't any issues after 3 years.
They are great and powerful Macs, but I will say your mileage before a major repair may vary.
I love my iMac. It's wonderful and a BIG upgrade for me. It's very fast with a full compliment of RAM
If you buy one, also buy Apple Care with the extended 3 year plan.
I bought used through a reseller that has an initialy 6 month warranty that I can only pay/extend out for a year after the initial period.