If you are serious about intending to do your own repairs or swap-outs, better at the very least have a strong back and lots of patience. the 'pro' of having everything housed on the back of a monitor quickly becomes a 'con' when you have to open it up for any reason. You have to manhandle the thing - safely - into a screen-down position to open it up, and then haul it up again when you're done.
But to begin with, you need to realize that companies that offer on-site warranty repair, and may have some sort of return/exchange policy for laptops (wherein if the system cannot reasonably repaired at your home or office they offer to pull it in to their repair depot) do not offer such an option for All-in-Ones (or Desktops) and you will pretty much need to keep making yourself available for either over-the-phone troubleshooting or for subsequent on-site repairs until the problem is resolved. Not a whole lot of customers actually like the repair depot option, but for nebulous, unidentifiable problems that cannot be narrowed down by troubleshooting, it does save a lot of headaches in the long run. But that's not usually an option unless the system in question is a laptop.
Every All-in-One is designed internally very differently - and many of the designs are far from easy to figure out just by popping off the back cover. In fact there are some models where perhaps the toughest thing to figure out is HOW to get the back off to begin with. Seriously - if you can't find an online manual or youtube video clearly showing you how (and there are some models where that info isn't either available or understandable) you run the risk of damaging the back cover and possibly even one or two internal components.
I've been doing on-site warranty repairs for 6 years now and I cringe whenever I see a work order come through for an AiO. Some are very simple and some are tougher to deal with than anything else I can think of. And what I said earlier about having a strong back - they're heavy and very awkward to maneuver, so I meant that from personal experience. You don't want to move it around if you can avoid it, and you should definitely make sure you have sufficient room for it wherever you intend to use it.
As far as doing your own swap-outs are concerned, simple components such as the hard drive and memory are one thing, and usually are pretty simple to handle. But worst-case scenario, if the screen goes on you, in most cases you're in for a whole lot of fun, and would be much better off paying someone else with (hopefully) experience to deal with it.
I'm not trying to tell you they're all negatives with no benefits - but I want you to be aware of the downside in addition to the positives, which you've already pointed out (the simplicity of design, from the user's perspective). When you asked about how easy it might be to replace a component if it went bad, I figured you should know that it really all depends on the model you settle on; most are easy to open up, and the main CRUs (customer replaceable units) are usually relatively easy to access. But I would definitely recommend getting an extended warranty if you do buy one, just so you don't have to wrestle with it yourself if something goes wrong down the road.